goprotobuf: Drop unused argument to enc_struct/size_struct.
goprotobuf: Drop unused argument to enc_struct/size_struct.

LGTM=djd
R=djd
CC=golang-codereviews
https://codereview.appspot.com/177110043

tamird/protobuf-2,trdata/protobuf,fuchsia-mirror/third_party-github.com-golang-protobuf,qinlodestar/protobuf,kung-foo/protobuf,mwitkow-io/protobuf,fuchsia-mirror/third_party-github.com-golang-protobuf,velocity-toolkit/protobuf-1,golang/protobuf,skyportsystems/go-protobuf,trdata/protobuf,adnaan/protobuf,ekle/protoc-gen-goweb_test,linkedin-inc/protobuf,hailocab/protobuf,daemonl/protobuf,francoishill/protobuf,eduardiazf/protobuf,tamird/protobuf-2,golang/protobuf,ekle/protoc-gen-goweb,csuideal/protobuf,jrbarron/protobuf,jeremyletang/protobuf,skyportsystems/go-protobuf,jmptrader/protobuf-1,aoeu/protobuf,alskjstl/protobuf,ArtemTeleshev/go-protobuf