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

[irfanuddinahmad]

Summary

Removes the built-in HTML XBlock from xmodule/html_block.py 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_HTML_BLOCK was set to True.

Deleted:

• _BuiltinHtmlBlockMixin class (~330 LOC) — built-in HTML mixin
• _BuiltInHtmlBlock class — built-in HTML block
• HtmlBlockMixin = None / reset_Mixin() — shim for the mixin
• HtmlBlock = None / reset_class() — shim for HtmlBlock
• xmodule/static/css-builtin-blocks/HtmlBlockDisplay.css
• All unused imports from html_block.py

Kept:

• AboutBlock, StaticTabBlock, CourseInfoBlock — still live in html_block.py; now inherit from xblocks_contrib.html.HtmlBlockMixin directly
• HtmlBlockEditor webpack entry — still loaded by the instructor dashboard
• xmodule/js/src/html/edit.js, HtmlBlockEditor.css — referenced by instructor dashboard template

Modified:

• openedx/envs/common.py — removed USE_EXTRACTED_HTML_BLOCK toggle
• pyproject.toml — removed conflicting html entrypoint
• webpack.builtinblocks.config.js — removed HtmlBlockDisplay entry
• xmodule/tests/test_html_block.py — removed toggle-conditional test classes; kept CourseInfoBlockTestCase
• lms/djangoapps/courseware/tests/test_block_render.py — import HtmlBlock from xblocks_contrib.html

Note: xmodule/js/src/html/display.js and imageModal.js are still referenced by AnnotatableBlockDisplay in webpack (removed in a separate PR for the Annotatable block) and will be deleted once that PR merges.

Test plan

• Run pytest xmodule/tests/test_html_block.py — CourseInfoBlockTestCase should pass
• Run pytest lms/djangoapps/courseware/tests/test_block_render.py — no HtmlBlock import errors
• Verify HTML XBlocks render in a live course (served by xblocks-contrib)
• Verify AboutBlock, StaticTabBlock, CourseInfoBlock still function in course structure
• Open instructor dashboard course updates — TinyMCE editor should still load (HtmlBlockEditor bundle retained)

🤖 Generated with Claude Code

Closes openedx/public-engineering#536

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