grpc: Reduce size of objects in memory (#368)

Given that we're not using any of the XXX_ fields (for example to check
for unknown fields), and we don't need golang/protobuf compatibility as
we're using gogoproto everywhere, we can tune the code generator to not
output these `XXX_*` fields and save 28 bytes for each decoded object we
keep in memory.

Signed-off-by: Adrian Serrano <adrisr83@gmail.com>