protoc-gen-go: drop golden test

Doesn't work in CI, since we don't have protoc available. Only tests the
gRPC generator, which is moving to the gRPC repo soon anyway. Just drop
it.

Change-Id: Ie0c5ddf1cc1ab4268838f3ea18ca2410cfe6d698
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/220497
Reviewed-by: Joe Tsai <[email protected]>

Author: neild