refactor: Replace videosequence & problemset (#29905)

`videosequence` and `problemset` have been replaced with `sequential`.

`problemset` and `videosequence` are old-but-not-entirely-unused aliases to the `sequential` block type (in Studio-speak, "Subsection").
Since [these block types have been removed from the 6 courses that used them](https://openedx.atlassian.net/browse/DEPR-151?focusedCommentId=588197), this ticket removes the support for the `problemset` and `videosequence` block-types.

For more information, see ticket: [DEPR-151](https://openedx.atlassian.net/browse/DEPR-151)

cms/djangoapps/contentstore/outlines.py

@@ -245,9 +245,7 @@ def _make_section_data(section, unique_sequences):
     if error:
         section_errors.append(error)
 
-    # We haven't officially killed off problemset and videosequence yet, so
-    # treat them as equivalent to sequential for now.
-    valid_sequence_tags = ['sequential', 'problemset', 'videosequence']
+    valid_sequence_tags = ['sequential']
     sequences_data = []
 
     for sequence in section.get_children():

cms/djangoapps/contentstore/tests/test_outlines.py

@@ -232,12 +232,12 @@ class OutlineFromModuleStoreTestCase(ModuleStoreTestCase):
             )
             ItemFactory.create(
                 parent=section_1,
-                category='problemset',
+                category='sequential',
                 display_name="pset_seq"
             )
             ItemFactory.create(
                 parent=section_1,
-                category='videosequence',
+                category='sequential',
                 display_name="video_seq"
             )