Remove `beta` from gRPC imports

[bjwatson]

What:

Change imports of grpc.beta... to grpc....

Why:

gRPC is almost GA, and gcloud has already made this change: googleapis/google-cloud-python#2149

[dhermes]

Thanks for filing @bjwatson. There were a few hairy parts that weren't just renames, e.g. grpc.secure_channel takes ('host:port', channel_creds) while grpc.beta.implementations.secure_channel takes (host, port, channel_credentials) (though the beta implementation calls the stable one).

[geigerj]

@dhermes Thanks for the heads-up on grpc.secure_channel. From the grpc source code, it looks like all other grpc.beta functions that we currently depend on are currently actually implemented as aliases to their non-beta equivalents, so should be pretty easy.

[dhermes]

Sure thing. I did a lot of grpc.foo is grpc.beta.foo and was pretty happy to find the same thing

[geigerj]

Status update: changing GAX requires updating generated code, which itself requires changing our tooling to use grpcio-tools instead of protoc + grpc_python_plugin. That change is waiting on a workaround to grpc/grpc#7857.