chore: remove obsolete proto folder (#359)

* chore: remove obsolete proto folder

* Update UPGRADING.md

* Restore link to sample annotation, but make it a permalink.

---------

Co-authored-by: meredithslota <meredithslota@google.com>
Co-authored-by: Chalmer Lowe <chalmer.lowe@gmail.com>
Co-authored-by: Victor Chudnovsky <vchudnov@google.com>

Author: 4 people

packages/google-cloud-build/UPGRADING.md

@@ -68,7 +68,7 @@ In `google-cloud-build<2.0.0`, parameters required by the API were positional pa
 
 In the 2.0.0 release, all methods have a single positional parameter `request`. Method docstrings indicate whether a parameter is required or optional.
 
-Some methods have additional keyword only parameters. The available parameters depend on the [`google.api.method_signature` annotation](https://github.com/googleapis/googleapis/blob/master/google/devtools/cloudbuild/v1/cloudbuild.proto#L82) specified by the API producer.
+Some methods have additional keyword-only parameters. The available parameters depend on the `google.api.method_signature` annotations provided by the API producer ([example](https://github.com/googleapis/googleapis/blob/6f5d107aadb4782b71c530f4dc17ec86c922d8a9/google/devtools/cloudbuild/v1/cloudbuild.proto#L93)).
 
 
 **After:**