refactor: slash-and-burn Annotatable BuiltIn XBlock

[irfanuddinahmad]

Summary

Removes the built-in Annotatable XBlock and all associated assets. The extracted xblocks_contrib.annotatable.AnnotatableBlock has been the active implementation since USE_EXTRACTED_ANNOTATABLE_BLOCK = True.

Step 3 of 8 in the Slash-n-Burn Built-In XBlocks epic.

Note: xmodule/annotator_mixin.py is intentionally kept — it is still used by cms/djangoapps/contentstore/courseware_index.py and xmodule/util/misc.py for the html_to_text utility function and is not part of this removal.

Changes

• Remove USE_EXTRACTED_ANNOTATABLE_BLOCK toggle from openedx/envs/common.py
• Remove annotatable entrypoint from pyproject.toml
• Delete xmodule/annotatable_block.py
• Delete xmodule/js/src/annotatable/display.js, JS spec, and HTML fixture
• Delete lms/templates/annotatable.html
• Delete xmodule/static/css-builtin-blocks/AnnotatableBlock{Display,Editor}.css
• Remove AnnotatableBlockDisplay / AnnotatableBlockEditor webpack entries
• Delete xmodule/tests/test_annotatable_block.py
• Update stale AnnotatableBlock reference in xblock_serializer/utils.py docstring

Test plan

• Run xmodule unit tests: pytest xmodule/tests/
• Verify Annotatable block still renders in a course (smoke test)
• Confirm webpack build succeeds: npm run build
• Confirm annotator_mixin.html_to_text still works (used by search indexing)

Closes openedx/public-engineering#535

Closes openedx/public-engineering#535

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