remove common-grpc logic that could be pulled in to gax #556
Description
This is a follow up to @alexander-fenster's comments:
https://github.com/googleapis/nodejs-bigtable/pull/554/files#r333816049, and
https://github.com/googleapis/nodejs-bigtable/pull/554/files#r333816370
On #554, I suggested that we pull in logic identical to @google-cloud/common-grpc, but work towards refactoring this out in future PRs (perhaps pulling more of the logic into gax).