cleanup(generator): avoid additional proto files flake

[dbolduc]

Related to #14518

Avoid flakes like: https://console.cloud.google.com/cloud-build/builds;region=us-east1/ce904a37-73d0-4e6d-8e41-ce3bb3cba307?project=936212892354

To convince myself that the changes work, and we do not end up with unmaintained files, I deleted the google/cloud/run/v2 directory before regenerating the code.

Which gives me an idea that we could do something like:

# Reset golden files
git grep -l "Generated by the Codegen C++ plugin" generator/integration_tests/golden | xargs rm

# Reset generated libraries
git grep -l "Generated by the Codegen C++ plugin" google/ | xargs rm

to make sure that generated files continue to be tracked by the generator.

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This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.59%. Comparing base (a86e887) to head (33c1de7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #14520   +/-   ##
=======================================
  Coverage   93.58%   93.59%           
=======================================
  Files        2316     2316           
  Lines      207108   207108           
=======================================
+ Hits       193826   193836   +10     
+ Misses      13282    13272   -10     

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