MAINT Remove requirements files

[adam2392]

Changes proposed in this pull request:

• Removes unnecessary requirements files
• build and style are still there because pip install .[style] will also install scikit-tree, which we don't need in the GH actions. Same for build.

Before submitting

• I've read and followed all steps in the Making a pull request
  section of the CONTRIBUTING docs.
• I've updated or added any relevant docstrings following the syntax described in the
  Writing docstrings section of the CONTRIBUTING docs.
• If this PR fixes a bug, I've added a test that will fail without my fix.
• If this PR adds a new feature, I've added tests that sufficiently cover my new functionality.

After submitting

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.05%. Comparing base (8ceada5) to head (31bd2d7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #240      +/-   ##
==========================================
+ Coverage   89.94%   90.05%   +0.11%     
==========================================
  Files          54       54              
  Lines        5050     5050              
==========================================
+ Hits         4542     4548       +6     
+ Misses        508      502       -6     

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