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/encoding/testprotos/pb2/test.pb.go b/encoding/testprotos/pb2/test.pb.go
index 38670db..49cf8e8 100644
--- a/encoding/testprotos/pb2/test.pb.go
+++ b/encoding/testprotos/pb2/test.pb.go
@@ -791,7 +791,7 @@
 	OptInt32        *int32  `protobuf:"varint,2,opt,name=opt_int32,json=optInt32" json:"opt_int32,omitempty"`
 	sizeCache       protoimpl.SizeCache
 	unknownFields   protoimpl.UnknownFields
-	extensionFields protoimpl.ExtensionFields ``
+	extensionFields protoimpl.ExtensionFields
 }
 
 func (x *Extensions) Reset() {
@@ -878,7 +878,7 @@
 type MessageSet struct {
 	sizeCache       protoimpl.SizeCache
 	unknownFields   protoimpl.UnknownFields
-	extensionFields protoimpl.ExtensionFields `protobuf_messageset:"1"`
+	extensionFields protoimpl.ExtensionFields
 }
 
 func (x *MessageSet) Reset() {
@@ -952,7 +952,7 @@
 type FakeMessageSet struct {
 	sizeCache       protoimpl.SizeCache
 	unknownFields   protoimpl.UnknownFields
-	extensionFields protoimpl.ExtensionFields ``
+	extensionFields protoimpl.ExtensionFields
 }
 
 func (x *FakeMessageSet) Reset() {
@@ -1389,7 +1389,7 @@
 		ExtendedType:  (*MessageSet)(nil),
 		ExtensionType: (*FakeMessageSetExtension)(nil),
 		Field:         50,
-		Name:          "pb2.",
+		Name:          "pb2.message_set_extension",
 		Tag:           "bytes,50,opt,name=message_set_extension",
 		Filename:      "pb2/test.proto",
 	},
@@ -1461,7 +1461,7 @@
 		ExtendedType:  (*MessageSet)(nil),
 		ExtensionType: (*MessageSetExtension)(nil),
 		Field:         10,
-		Name:          "pb2.MessageSetExtension",
+		Name:          "pb2.MessageSetExtension.message_set_extension",
 		Tag:           "bytes,10,opt,name=message_set_extension",
 		Filename:      "pb2/test.proto",
 	},