refactor: migrate ENABLE_INSTRUCTOR_EMAIL and REQUIRE_COURSE_EMAIL_AUTH off FEATURES-as-dict

Three bulk_email test classes carried
`@patch.dict(settings.FEATURES, {'ENABLE_INSTRUCTOR_EMAIL': True,
'REQUIRE_COURSE_EMAIL_AUTH': False})`. Neither FEATURES key is read
anywhere in the platform or the installed packages anymore — bulk email
enablement is now governed by the BulkEmailFlag waffle flag and the
CourseAuthorization model — so these class-level overrides are vestigial
no-ops (and, being class-level `patch.dict`, only ever wrapped the
`test_*` methods, not setUp).

Both flags sit in a single decorator dict, so they migrate together:
convert to `@override_settings(ENABLE_INSTRUCTOR_EMAIL=True,
REQUIRE_COURSE_EMAIL_AUTH=False)`. Behavior is unchanged (still unread);
this just removes the FEATURES-dict usage. The classes subclass
SharedModuleStoreTestCase, so the class-level override_settings is valid.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>