Revert "Merge branch 'an-upgrade-grpc' into 'master'"

This reverts merge request !972 (merged), which upgrade grpc-go from 1.9.x to 1.16.x.

I don't like doing this but I think it's the best thing to do.

• !972 (merged) deleted code gitaly/client that is being modified in !932 (merged)
• the deleted code is library code that gets vendored into gitlab-shell, gitaly-ssh and gitlab-workhorse
• the deleted code has very poor test coverage (this is being addressed in !968 (merged))
• because of the poor coverage of the deleted code there is a risk of regressions
• worst case failure is that gitlab-shell and gitlab-workhorse can no longer connect to Gitaly with tcp:// addresses, and that we only find out about this in staging, delaying a GitLab release

Considering that we are working on improved test coverage in !968 (merged), I think we should finish that first and then try to re-apply !972 (merged). Even in the happy scenario where !972 (merged) does not break connectivity, we are better off having the tests from !968 (merged) tell us it's OK.