feat!: remove most Old Mongo functionality (#31134)

This commit leaves behind just enough Old Mongo (DraftModulestore)
functionality to allow read-only access to static assets and the
root CourseBlock. It removes:

* create/update operations
* child/parent traversal
* inheritance related code

It also removes or converts tests for this functionality.

The ability to read from the root CourseBlock was maintained for
backwards compatibility, since top-level course settings are often
stored here, and this is used by various parts of the codebase,
like displaying dashboards and re-building CourseOverview models.

Any attempt to read the contents of a course by getting the
CourseBlock's children will return an empty list (i.e. it will look
empty).

This commit does _not_ delete content on MongoDB or run any sort of
data migration or cleanup.

xmodule/tests/test_course_metadata_utils.py

@@ -30,7 +30,6 @@ from xmodule.modulestore.tests.utils import (
 )
 
 _TODAY = datetime.now(utc)
-_LAST_MONTH = _TODAY - timedelta(days=30)
 _LAST_WEEK = _TODAY - timedelta(days=7)
 _NEXT_WEEK = _TODAY + timedelta(days=7)
 
@@ -49,34 +48,9 @@ class CourseMetadataUtilsTestCase(TestCase):
 
         mongo_builder = MongoModulestoreBuilder()
         split_builder = VersioningModulestoreBuilder()
-        mixed_builder = MixedModulestoreBuilder([('mongo', mongo_builder), ('split', split_builder)])
+        mixed_builder = MixedModulestoreBuilder([('split', split_builder), ('mongo', mongo_builder)])
 
         with mixed_builder.build_without_contentstore() as (__, mixed_store):
-            with mixed_store.default_store('mongo'):
-                self.demo_course = mixed_store.create_course(
-                    org="edX",
-                    course="DemoX.1",
-                    run="Fall_2014",
-                    user_id=-3,  # -3 refers to a "testing user"
-                    fields={
-                        "start": _LAST_MONTH,
-                        "end": _LAST_WEEK,
-                        "enrollment_start": _LAST_MONTH,
-                        "enrollment_end": _LAST_WEEK
-                    }
-                )
-            with mixed_store.default_store('mongo'):
-                self.demo_course_enrollment_end = mixed_store.create_course(
-                    org="edX",
-                    course="DemoX.2",
-                    run="Fall_2014_1",
-                    user_id=-3,  # -3 refers to a "testing user"
-                    fields={
-                        "start": _LAST_MONTH,
-                        "end": _LAST_WEEK,
-                        "enrollment_end": _LAST_WEEK
-                    }
-                )
             with mixed_store.default_store('split'):
                 self.html_course = mixed_store.create_course(
                     org="UniversityX",
@@ -145,11 +119,6 @@ class CourseMetadataUtilsTestCase(TestCase):
 
         function_tests = [
             FunctionTest(clean_course_key, [
-                # Test with a Mongo course and '=' as padding.
-                TestScenario(
-                    (self.demo_course.id, '='),
-                    "course_MVSFQL2EMVWW6WBOGEXUMYLMNRPTEMBRGQ======"
-                ),
                 # Test with a Split course and '~' as padding.
                 TestScenario(
                     (self.html_course.id, '~'),
@@ -157,39 +126,26 @@ class CourseMetadataUtilsTestCase(TestCase):
                 ),
             ]),
             FunctionTest(url_name_for_block, [
-                TestScenario((self.demo_course,), self.demo_course.location.block_id),
                 TestScenario((self.html_course,), self.html_course.location.block_id),
             ]),
             FunctionTest(display_name_with_default_escaped, [
-                # Test course with no display name.
-                TestScenario((self.demo_course,), "Empty"),
                 # Test course with a display name that contains characters that need escaping.
                 TestScenario((self.html_course,), "Intro to html"),
             ]),
             FunctionTest(display_name_with_default, [
-                # Test course with no display name.
-                TestScenario((self.demo_course,), "Empty"),
                 # Test course with a display name that contains characters that need escaping.
                 TestScenario((self.html_course,), "Intro to <div>html</div>"),
             ]),
             FunctionTest(number_for_course_location, [
-                TestScenario((self.demo_course.location,), "DemoX.1"),
                 TestScenario((self.html_course.location,), "CS-203"),
             ]),
             FunctionTest(has_course_started, [
-                TestScenario((self.demo_course.start,), True),
                 TestScenario((self.html_course.start,), False),
             ]),
             FunctionTest(has_course_ended, [
-                TestScenario((self.demo_course.end,), True),
                 TestScenario((self.html_course.end,), False),
             ]),
             FunctionTest(is_enrollment_open, [
-                TestScenario((self.demo_course.enrollment_start, self.demo_course.enrollment_end,), False),
-                TestScenario((
-                    self.demo_course_enrollment_end.enrollment_start,
-                    self.demo_course_enrollment_end.enrollment_end
-                ), False),
                 TestScenario((self.html_course.enrollment_start, self.html_course.enrollment_end,), False),
                 TestScenario((
                     self.html_course_enrollment_start.enrollment_start,