Cleanup and remove deprecated RequestCache Django app

ARCH-223

common/djangoapps/student/models.py

@@ -50,6 +50,7 @@ from six import text_type
 from slumber.exceptions import HttpClientError, HttpServerError
 from user_util import user_util
 
+from edx_django_utils.cache import RequestCache
 import lms.lib.comment_client as cc
 from student.signals import UNENROLL_DONE, ENROLL_STATUS_CHANGE, ENROLLMENT_TRACK_UPDATED
 from lms.djangoapps.certificates.models import GeneratedCertificate
@@ -62,7 +63,6 @@ from courseware.models import (
 from enrollment.api import _default_course_mode
 
 from openedx.core.djangoapps.content.course_overviews.models import CourseOverview
-from openedx.core.djangoapps.request_cache import clear_cache, get_cache
 from openedx.core.djangoapps.site_configuration import helpers as configuration_helpers
 from openedx.core.djangoapps.xmodule_django.models import NoneToEmptyManager
 from openedx.core.djangolib.model_mixins import DeletableByUserValue
@@ -2069,7 +2069,7 @@ class CourseEnrollment(models.Model):
         """
         # before populating the cache with another bulk set of data,
         # remove previously cached entries to keep memory usage low.
-        clear_cache(cls.MODE_CACHE_NAMESPACE)
+        RequestCache(cls.MODE_CACHE_NAMESPACE).clear()
 
         records = cls.objects.filter(user__in=users, course_id=course_key).select_related('user')
         cache = cls._get_mode_active_request_cache()
@@ -2080,9 +2080,9 @@ class CourseEnrollment(models.Model):
     @classmethod
     def _get_mode_active_request_cache(cls):
         """
-        Returns the request-specific cache for CourseEnrollment
+        Returns the request-specific cache for CourseEnrollment as dict.
         """
-        return get_cache(cls.MODE_CACHE_NAMESPACE)
+        return RequestCache(cls.MODE_CACHE_NAMESPACE).data
 
     @classmethod
     def _get_enrollment_in_request_cache(cls, user, course_key):

common/djangoapps/student/roles.py

@@ -10,7 +10,7 @@ from collections import defaultdict
 from django.contrib.auth.models import User
 from opaque_keys.edx.django.models import CourseKeyField
 
-from openedx.core.djangoapps.request_cache import get_cache
+from openedx.core.lib.cache_utils import get_cache
 from student.models import CourseAccessRole
 
 log = logging.getLogger(__name__)

common/djangoapps/student/tests/test_email.py

@@ -24,6 +24,7 @@ from student.models import (
     UserProfile,
     get_retired_email_by_email
 )
+from openedx.core.lib.request_utils import safe_get_host
 from student.tests.factories import PendingEmailChangeFactory, RegistrationFactory, UserFactory
 from student.views import (
     SETTING_CHANGE_INITIATED,
@@ -33,7 +34,6 @@ from student.views import (
 )
 from student.views import generate_activation_email_context, send_reactivation_email_for_user
 from third_party_auth.views import inactive_user_view
-from util.request import safe_get_host
 from util.testing import EventTestMixin