feat: remove built-in Discussion XBlock implementation (slash-n-burn #37819)

[irfanuddinahmad]

Summary

Removes the built-in Discussion XBlock from xmodule/ as part of the Slash-n-Burn Built-In XBlocks epic (#37819). The extracted version in xblocks-contrib has been the active implementation since USE_EXTRACTED_DISCUSSION_BLOCK was set to True.

Deleted:

• xmodule/discussion_block.py — built-in _BuiltInDiscussionXBlock and shim
• xmodule/tests/test_discussion.py — built-in unit tests
• xmodule/js/src/discussion/display.js — block JS

Modified:

• openedx/envs/common.py — removed USE_EXTRACTED_DISCUSSION_BLOCK toggle
• pyproject.toml — removed conflicting discussion entrypoint
• lms/djangoapps/courseware/management/commands/dump_course_structure.py — import DiscussionXBlock from xblocks_contrib.discussion
• openedx/core/djangoapps/discussions/utils.py — import DiscussionXBlock from xblocks_contrib.discussion
• lms/djangoapps/courseware/tests/test_discussion_xblock.py — import DiscussionXBlock from xblocks_contrib.discussion

Note: lms/djangoapps/discussion/ (the forum service Django app) is NOT in scope for this removal.

Test plan

• Run pytest lms/djangoapps/courseware/tests/test_discussion_xblock.py — all tests pass
• Verify Discussion XBlocks in a live course still render (served by xblocks-contrib)
• Run the dump_course_structure management command on a course with discussion blocks — no import errors

🤖 Generated with Claude Code

Closes openedx/public-engineering#538

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