Sort out Go and Protobuf

[pongad]

This is a meta-issue so we can specify the order in which issues regarding Go and Protobuf should be resolved. The primary goal of the issue is so that I don't do anything to upset the gopher god.

• Document the existence of google/go-genproto in googleapis/googleapis README (Document that Go source files live in google/go-genproto googleapis#59)
• Add deprecation warning to googleapis/proto-client-go, it's superseded by google/go-genproto (https://github.com/googleapis/proto-client-go/pull/7)
• Delete Go-specific portions of googleapis/googleapis/Makefile (Remove Go-specific logic from Makefile googleapis#60)
• Update toolkit to point to google/go-genproto instead of github.com/googleapis/proto-client-go (issue: Import generated proto code from google.golang.org/genproto #312) (Redirect Go core proto path to google.golang.org/genproto/googleapis #327)
• Regenerate gcloud-golang with the updated toolkit (https://code-review.googlesource.com/#/c/5401/)
• Delete googleapis/proto-client-go

[pongad]

I reached out to @saicheems and he says the order looks reasonable. @broady @jba , can you verify?

[pongad]

@saicheems The regenerated logging client is merged. Can you help conform that googleapis/proto-client-go is safe to delete?

[saicheems]

Hmmm yeah, I think so!

[saicheems]

Oh wait, it's still part of gocloud-legacy... maybe we should leave it up until September 12th?

https://code.googlesource.com/gocloud-legacy/+/master

[saicheems]

If there are no objections, I'm going to delete the proto-client-go repo tonight.

[pongad]

@saicheems No objection from my side.

[saicheems]

Done