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

wfxiang08/goprotobuf,ruiaylin/goprotobuf,liumuqi/goprotobuf,adwinsky/goprotobuf