docs: remove deprecated acousticbrainz, absubmit, and beatport plugins

[Mo200227]

Description

Fixes #5780

This PR removes documentation for three plugins that are no longer functional:

• acousticbrainz — the AcousticBrainz service was permanently shut down
• absubmit — same upstream shutdown (submits to AcousticBrainz)
• beatport — deprecated as noted in issue docs: Identify and clean up references to broken or unmaintained plugins #5780

Changes

• Deleted docs/plugins/absubmit.rst
• Deleted docs/plugins/acousticbrainz.rst
• Deleted docs/plugins/beatport.rst
• Removed all references to these three plugins from docs/plugins/index.rst

To Do

• Documentation updated
• Changelog (not required for doc-only removal)
• Tests (no

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[Mo200227]

"Hi @snejus — this is my first contribution to beets. I've removed the deprecated acousticbrainz, absubmit, and beatport plugin docs as discussed in #5780. Happy to make any changes needed!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.04%. Comparing base (3438b8b) to head (ba03515).
⚠️ Report is 248 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6778      +/-   ##
==========================================
+ Coverage   72.60%   75.04%   +2.44%     
==========================================
  Files         162      163       +1     
  Lines       20810    20949     +139     
  Branches     3294     3300       +6     
==========================================
+ Hits        15109    15722     +613     
+ Misses       4977     4468     -509     
- Partials      724      759      +35     

see 75 files with indirect coverage changes

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[snejus]

Please rebase on master so that the CI runs. Note that I added this to 3.0.0 milestone since it can only be merged when we release a new major version. :)