Removes the following deprecated Prometheus metrics:

- coderd_api_workspace_latest_build_total (use coderd_api_workspace_latest_build instead)
- coderd_oauth2_external_requests_rate_limit_total (use coderd_oauth2_external_requests_rate_limit instead)

These metrics were deprecated in #12976 because gauge metrics should
avoid the `_total` suffix per Prometheus naming conventions.

Fixes #12999