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 <thebrokentoaster@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/cmd/protoc-gen-go/internal_gengo/main.go b/cmd/protoc-gen-go/internal_gengo/main.go
index 7081933..6338c44 100644
--- a/cmd/protoc-gen-go/internal_gengo/main.go
+++ b/cmd/protoc-gen-go/internal_gengo/main.go
@@ -57,7 +57,6 @@
 	protojsonPackage     goImportPath = protogen.GoImportPath("google.golang.org/protobuf/encoding/protojson")
 	protoreflectPackage  goImportPath = protogen.GoImportPath("google.golang.org/protobuf/reflect/protoreflect")
 	protoregistryPackage goImportPath = protogen.GoImportPath("google.golang.org/protobuf/reflect/protoregistry")
-	protoV1Package       goImportPath = protogen.GoImportPath("github.com/golang/protobuf/proto")
 )
 
 type goImportPath interface {