chore: drop generate-protos.js in favor of npm script

[pichlermarc]

Which problem is this PR solving?

See #6055

The script can be made to work on all platforms, but it's not necessary anymore since we only have one package that needs protos to be generated. Previously, we generated these for both the proto exporter base (which does not exist anymore) and the grpc exporter base which is why a script was easier to deal with. That functionality has been consolidated and moved to @opentelemetry/otlp-transformer in #4542, removing the need to generate code in the exporter package itself in the cleanup steps that followed that PR.

Supersedes #6055

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.21%. Comparing base (449cd8c) to head (94dabb9).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6134   +/-   ##
=======================================
  Coverage   95.21%   95.21%           
=======================================
  Files         316      316           
  Lines        9388     9390    +2     
  Branches     2168     2168           
=======================================
+ Hits         8939     8941    +2     
  Misses        449      449           

see 1 file with indirect coverage changes

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