chore: remove unused fossaignore

[mark-wiemer]

PR Checklist

• Addresses an existing open issue: fixes 📝 Docs: Remove 'license scan | failing' badge from README.md #5161 (close enough!)
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Took me a while to find what this file used to represent, now I can remove it with confidence!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.78%. Comparing base (c3f32ce) to head (1373cc8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5769      +/-   ##
==========================================
+ Coverage   88.74%   88.78%   +0.04%     
==========================================
  Files          66       66              
  Lines        4744     4744              
  Branches      976      976              
==========================================
+ Hits         4210     4212       +2     
+ Misses        534      532       -2     

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[JoshuaKGoldberg]

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