internal/cmd/generate-alias: avoid generating version markers

We control both v1 and v2.
The generated version markers are just a nuisance.

Change-Id: I0d3248b20414f6279a83f7159bc3a4493619ce36
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/191677
Reviewed-by: Damien Neil <[email protected]>

Author: dsnet

internal/cmd/generate-alias/main.go

@@ -86,6 +86,7 @@ func main() {
 	check(err)
 	for _, file := range gen.Files {
 		if file.Generate {
+			gengo.GenerateVersionMarkers = false
 			gengo.GenerateFile(gen, file)
 		}
 	}