Duplicate integration tests for gRPC vs. REST

I think the `google/cloud/storage/tests/grpc_*_integration_test.cpp` are duplicating work. OTOTH, they are easier to follow, so maybe we should keep those are remove the (portions of) the older integration tests.

Start by comparing google/cloud/storage/tests/grpc_*_integration_test.cpp with the older storage integration tests to identify overlapping coverage and the portions under discussion. Review how the integration tests are organized and verify that the retained tests still cover the intended gRPC and REST behavior; done means duplication is reduced without losing necessary coverage.

Written by the indexing model from the issue text.