cmd/protoc-gen-go: remove unused protoV1Package variable
cmd/protoc-gen-go: remove unused protoV1Package variable

The logic that formerly depended on this was deleted in
https://golang.org/cl/259901. This is dead code now.

Updates golang/protobuf#1077

Change-Id: Ieb1b14243d9503b1baac7026c6ce226923873405
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/260817
Trust: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go