cmd/protoc-gen-go: remove stale comment
cmd/protoc-gen-go: remove stale comment

Change-Id: I88c5536c29602206657cbeab81347c872732da56
Reviewed-on: https://go-review.googlesource.com/c/156198
Reviewed-by: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go