Remove second test (regional latency) from GCS object_storage_service

The object_storage_service benchmark for GCS tests one-byte-RW (latency) for a regional bucket after testing a multi-region bucket.

My suggestion from discussion in #750 is to remove this second test and instead allow users to make use of `--object_storage_class` (proposed in #836), `--object_storage_region` and `--object_storage_scenario` to get whatever benchmark combo they want.