cleanup: `bigtable/benchmarks` is clang-tidy clean after #4138 (#4231)
cleanup: `bigtable/benchmarks` is clang-tidy clean after #4138 (#4231)

The presubmit tests won't verify that, but I'll assert it,
and we'll learn whether I'm wrong as future edits are made.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp