internal/proto: remove use of MessageType.GoType

Remove use in preparation for dropping the GoType method entirely.

Change-Id: Ia6df16acbf30c77b26c33eacf6d68fb59d776ca5
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/198817
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

internal/proto/registry.go

@@ -285,7 +285,7 @@ func MessageType(s messageName) reflect.Type {
 	var t reflect.Type
 	mt, _ := protoregistry.GlobalTypes.FindMessageByName(pref.FullName(s))
 	if mt != nil {
-		t = mt.GoType()
+		t = reflect.TypeOf(mt.Zero().Interface())
 	}
 	// TODO: Support retrieving Go map types for map entry messages?