Fox/remove web fragments

[Kelketek]

This PR removes web-fragments as an independent package, now that it's included in XBlock's packaging directly.

See this PR.

CI testing should work well enough to validate, but you can test by mounting this repo in Tutor, installing it, and then using some of the blocks to verify they can still load their contents.

Ticket: openedx/web-fragments#309

Merge checklist:
Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

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