Revert: Cannot rerun courses - authz role assignment expects rerun to already exist (#38840) (#38973)

* Revert "fix: Cannot rerun courses - authz role assignment expects rerun to already exist (#38840) (#38858)"

This reverts commit ceb6a18eb3e0c886ae92cce673eb820bbf343c25.

* fix: remove unused imports left after revert

* fix: remove stray pre-38840 test reintroduced by conflict resolution

test_create_course_disabled_by_flag and its 'authorized_staff_client'
fixture belonged to the old TestCourseHandlerAuthz class, which an
unrelated refactor (not #38840) had already replaced with the leaner
TestCourseHandlerStaffAccess before #38840 was even merged. The revert's
merge conflict was between that refactor and #38840's insertion point,
not a real need to restore this method.

* chore: upgrade openedx-authz to 1.22.0

1.22.0 ships the fix for openedx-authz#352 (role assignment on a
course/library scope before its CourseOverview/ContentLibrary exists),
which this revert depends on.

* test: restore TestCourseRerunAuthz/TestCourseRerunLegacy coverage

Per review feedback from @mariajgrimaldi: these test classes were
removed as part of the literal revert of #38840, but they don't
actually test the removed workaround itself — they test that rerun
succeeds and grants correct access with authz enabled/disabled, which
is exactly the behavior this revert needs to keep working now that
openedx-authz#352 is implemented.

Verified locally against the real devstack (openedx-authz 1.22.0+):
all 7 tests pass unmodified against the reverted code. Cleaned up the
docstrings that referenced the now-removed conditional add_instructor
skip and created_user fallback, since add_instructor runs unconditionally
pre-task again.