Add Softmax ranker and remove legacy code

[mdekstrand]

This does 2 things, unfortunately:

• Implement a Softmax ranker to replace the Gumbel-based Plackett-Luce ranker in the old code
• Remove legacy algorithms, batch inference, and evaluation support and its tests

There is still some legacy code to remove, but this gets rid of a lot of it.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 92.48%. Comparing base (c08f333) to head (56c132b).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
lenskit/lenskit/util/test.py	82.75%	5 Missing ⚠️
lenskit/lenskit/basic/random.py	88.88%	3 Missing ⚠️
lenskit/lenskit/splitting/__init__.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #529      +/-   ##
==========================================
- Coverage   93.18%   92.48%   -0.71%     
==========================================
  Files         101       87      -14     
  Lines        7063     5312    -1751     
==========================================
- Hits         6582     4913    -1669     
+ Misses        481      399      -82     

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