Fix imports, remove unused import

[0x29a]

A few minor warnings fixed, and one unused import removed.

1. ./manage.py lms shell -c "print('hello')"

2020-11-19 09:36:40,680 WARNING 2788 [py.warnings] [user None] [ip None] warnings.py:109 - /edx/app/edxapp/edx-platform/openedx/core/djangoapps/user_authn/cookies.py:25: DeprecatedEdxPlatformImportWarning: Importing student.models instead of common.djangoapps.student.models is deprecated
  from student.models import CourseEnrollment

2020-11-19 09:36:40,680 WARNING 2788 [py.warnings] [user None] [ip None] warnings.py:109 - /edx/app/edxapp/edx-platform/openedx/core/djangoapps/user_authn/cookies.py:26: DeprecatedEdxPlatformImportWarning: Importing util.json_request instead of common.djangoapps.util.json_request is deprecated
  from util.json_request import JsonResponse

2. python manage.py lms migrate

2020-11-19 09:52:25,431 WARNING 2969 [py.warnings] [user None] [ip None] warnings.py:109 - /edx/app/edxapp/edx-platform/openedx/core/djangoapps/content/learning_sequences/api/processors/content_gating.py:7: DeprecatedEdxPlatformImportWarning: Importing util.milestones_helpers instead of common.djangoapps.util.milestones_helpers is deprecated
  from util import milestones_helpers

3. pytest --pdbcls pudb.debugger:Debugger --pdb --capture=no lms/djangoapps/instructor_task:

lms/djangoapps/instructor_task/tests/test_tasks_helper.py: 20 warnings
lms/djangoapps/instructor_task/tests/test_integration.py: 20 warnings
  /edx/app/edxapp/venvs/edxapp/lib/python3.8/site-packages/edx_django_utils/monitoring/code_owner/middleware.py:23: DeprecationWarning: Use 'edx_django_utils.monitoring.CodeOwnerMonitoringMiddleware' in place of 'edx_django_utils.monitoring.code_owner.middleware.CodeOwnerMonitoringMiddleware'.
    warnings.warn(msg, DeprecationWarning)

and

lms/djangoapps/instructor_task/tests/test_tasks_helper.py: 20 warnings
lms/djangoapps/instructor_task/tests/test_integration.py: 20 warnings
  /edx/app/edxapp/venvs/edxapp/lib/python3.8/site-packages/edx_django_utils/monitoring/middleware.py:23: DeprecationWarning: Use 'edx_django_utils.monitoring.CachedCustomMonitoringMiddleware' in place of 'edx_django_utils.monitoring.middleware.CachedCustomMonitoringMiddleware'.
    warnings.warn(msg, DeprecationWarning)

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[0x29a]

@natabene, this is ready for edX review.

[nedbat]

Thanks!

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EdX Release Notice: This PR has been deployed to the staging environment in preparation for a release to production.

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EdX Release Notice: This PR has been deployed to the production environment.