cmd/protoc-gen-go: remove name mangling for MessageSet extensions

MessageSet are a proto1 feature only used by Google.
We no longer rely on the name mangling logic performed here,
so remove it.

Change-Id: I5d66ebd86875894632f0d0c1e9816ae47ee0d5f4
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/242657
Reviewed-by: Herbie Ong <herbie@google.com>

cmd/protoc-gen-go/internal_gengo/main.go

@@ -17,7 +17,6 @@ import (
 	"unicode/utf8"
 
 	"google.golang.org/protobuf/compiler/protogen"
-	"google.golang.org/protobuf/internal/encoding/messageset"
 	"google.golang.org/protobuf/internal/encoding/tag"
 	"google.golang.org/protobuf/internal/genid"
 	"google.golang.org/protobuf/internal/version"
@@ -735,12 +734,6 @@ func genExtensions(g *protogen.GeneratedFile, f *fileInfo) {
 
 	g.P("var ", extensionTypesVarName(f), " = []", protoimplPackage.Ident("ExtensionInfo"), "{")
 	for _, x := range f.allExtensions {
-		// For MessageSet extensions, the name used is the parent message.
-		name := x.Desc.FullName()
-		if messageset.IsMessageSetExtension(x.Desc) {
-			name = name.Parent()
-		}
-
 		g.P("{")
 		g.P("ExtendedType: (*", x.Extendee.GoIdent, ")(nil),")
 		goType, pointer := fieldGoType(g, f, x.Extension)
@@ -749,7 +742,7 @@ func genExtensions(g *protogen.GeneratedFile, f *fileInfo) {
 		}
 		g.P("ExtensionType: (", goType, ")(nil),")
 		g.P("Field: ", x.Desc.Number(), ",")
-		g.P("Name: ", strconv.Quote(string(name)), ",")
+		g.P("Name: ", strconv.Quote(string(x.Desc.FullName())), ",")
 		g.P("Tag: ", strconv.Quote(fieldProtobufTagValue(x.Extension)), ",")
 		g.P("Filename: ", strconv.Quote(f.Desc.Path()), ",")
 		g.P("},")