error-prone UnusedVariable check raises false positives for StateId and TimerId usages

See https://github.com/google/error-prone/issues/2668. The next release of error-prone will contain https://github.com/google/error-prone/pull/2669 and https://github.com/google/error-prone/pull/2676, which will help us.

Until then we've had to suppress the check in several places. This jira tracks upgrading to a new error-prone once released and removing the suppressions.

Imported from Jira [BEAM-13271](https://issues.apache.org/jira/browse/BEAM-13271). Original Jira may contain additional context.
Reported by: bhulette.

Review the linked Error Prone issue and pull requests 2669 and 2676 first, then locate the existing suppressions for the UnusedVariable check in the Beam repository. Upgrade Error Prone after the fix is released, remove the suppressions, and verify that the project builds and the affected checks pass.

Written by the indexing model from the issue text.