cmd/protoc-gen-go: remove MessageSet hackery

The encoding/prototext and encoding/protojson are implemented entirely
in terms of protobuf reflection, which side-steps this information.
Remove the hacks in the generator to special-case MessageSet.

Change-Id: I708c4636b77672545a103b7ab686f103b9dfc514
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/185240
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/internal/testprotos/test/test.pb.go b/internal/testprotos/test/test.pb.go
index e044c53..bae3e17 100644
--- a/internal/testprotos/test/test.pb.go
+++ b/internal/testprotos/test/test.pb.go
@@ -1174,7 +1174,7 @@
 type TestAllExtensions struct {
 	sizeCache       protoimpl.SizeCache
 	unknownFields   protoimpl.UnknownFields
-	extensionFields protoimpl.ExtensionFields ``
+	extensionFields protoimpl.ExtensionFields
 }
 
 func (x *TestAllExtensions) Reset() {