chore: simplify googleapis overlay BUILD (googleapis/google-cloud-cpp-spanner#1470)
chore: simplify googleapis overlay BUILD (googleapis/google-cloud-cpp-spanner#1470)

Part of https://github.com/googleapis/google-cloud-cpp/issues/3838
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp