refactor: 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.

Epic Step 1: Post-extraction commit review

Reviewed commits to xmodule/discussion_block.py after USE_EXTRACTED_DISCUSSION_BLOCK was set to True (2026-04-15 · c534f5754f). Only 1 post-toggle commit: lint-amnesty removal (cosmetic). A legacy-attribute bug fix (d71f81312d) landed 2026-04-14 — the day before the toggle — and an equivalent independent fix is already in xblocks-contrib (0dd64cd, 2026-04-02). No functional bug fixes or features to port.

Test plan

Testing scope: The xblocks-contrib implementation's behavioral correctness was fully validated during the extraction phase (when USE_EXTRACTED_* was set to True). These slash-n-burn PRs only delete dead built-in code — no behavioral change is possible. The only regression risk is that an entrypoint, webpack bundle, or import path is broken, which surfaces as a block failing to load. Basic load verification across LMS, CMS course, and CMS library is sufficient.

Smoke tests require a live Tutor environment and cannot be pre-checked in CI. Please verify during staging.

• Block loads and renders in LMS (student view)
• Block loads and renders in CMS course unit
• Block loads and renders in CMS V2 Library

Automated checks (CI):

• Run pytest lms/djangoapps/courseware/tests/test_discussion_xblock.py — all tests pass
• Run the dump_course_structure management command on a course with discussion blocks — no import errors

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