cmd/protoc-gen-go: remove XXX_OneofWrappers
cmd/protoc-gen-go: remove XXX_OneofWrappers

Change-Id: I31a311c9ea24e959d5d641c66c4ee77f0c98a2ed
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/186917
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go