remove unused command fetch rbfe-showcase

[atravitz]

currently we don't even show rbfe-showcase as an option:

> openfe fetch -h

  Fetch the given resource. Some resources require internet; others are built-
  in.

Options:
  -h, --help  Show this message and exit.

Tutorials Commands:
  rbfe-tutorial          CLI and Python tutorial on relative binding free
                         energies [requires internet]
  rbfe-tutorial-results  Results package to follow-up the rbfe-tutorial

And we just removed these unused files from ExampleNotebooks: OpenFreeEnergy/ExampleNotebooks#269

Checklist

• All new code is appropriately documented (user-facing code must have complete docstrings).
• Added a news entry, or the changes are not user-facing.
• Ran pre-commit by making a comment with pre-commit.ci autofix before requesting review.

Manual Tests: these are slow so don't need to be run every commit, only before merging and when relevant changes are made (generally at reviewer-discretion).

• GPU integration tests
• example notebook testing
• packaging tests: run this for any large feature PRs or PRs that add test data.

Developers certificate of origin

• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[github-actions]

No API break detected ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.15%. Comparing base (f491184) to head (17c77fa).
⚠️ Report is 66 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1732      +/-   ##
==========================================
- Coverage   95.47%   93.15%   -2.32%     
==========================================
  Files         187      187              
  Lines       16244    16240       -4     
==========================================
- Hits        15509    15129     -380     
- Misses        735     1111     +376     

Flag	Coverage Δ
fast-tests	93.15% <100.00%> (?)
slow-tests	?

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