refactor: migrate ENABLE_TEXTBOOK and ENABLE_EDXNOTES test overrides off FEATURES-as-dict

Both flags' production readers already read flat settings (courseware/plugins.py,
courseware/tabs.py, edxnotes/decorators.py, cms course_metadata.py). One tabs test still
enabled them via @patch.dict(settings.FEATURES, {...}) alongside an
@override_settings(ENABLE_DISCUSSION_SERVICE=True); fold both flags into that
override_settings so no settings.FEATURES usage remains.

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