Google authentication cleanup.
Google authentication cleanup.

- Removing legacy way to authenticate with Google.
- Java and Python will need to update their examples to use "scopeless"
  creds.
- Fixes #180.

avx2/avx2.github.io,avx2/avx2.github.io,wildebeestdev/grpc.github.io,wildebeestdev/grpc.github.io,wildebeestdev/grpc.github.io,avx2/avx2.github.io