Remove unused Event.for_subject scope

[dreikanter]

Changes:

• Remove the unused Event.for_subject scope.
• Remove the test that only exercised that scope.

Why:

The scope has no application callers and adds an unnecessary model API.

References:

• Related issue: Review the code structure #1010 (F-108)

Checks:

• Remaining event model coverage is unchanged.
• GitHub Actions will verify the branch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.55%. Comparing base (ce0547a) to head (5e3c818).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1089      +/-   ##
==========================================
- Coverage   97.55%   97.55%   -0.01%     
==========================================
  Files         355      355              
  Lines        8358     8357       -1     
==========================================
- Hits         8154     8153       -1     
  Misses        204      204              

Files with missing lines	Coverage Δ
app/models/event.rb	100.00% <ø> (ø)

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