Remove obsolete GCP secret access from mobile CI workflow

The mobile app code analysis workflow is still authenticating to Google Cloud and fetching Firebase-related secrets, but the project has migrated to Azure and no longer uses GCP.

Observed failure:
- Workflow: `.github/workflows/dart-code-metrics.yaml`
- Failing step: `Add firebase options files`
- Error: `gcloud.secrets.versions.access ... NOT_FOUND` for `sta-key-mobile-firebase-options-dev`

Requested cleanup:
- Remove or replace GCP auth and `gcloud secrets` usage in the mobile CI workflow.
- Update the workflow to use the current Azure-based setup, or remove the step entirely if it is no longer needed.
- Verify the workflow still supports mobile code analysis without GCP dependencies.