all: remove unused purego support

fixes golang/protobuf#1640

Change-Id: I0e8f2df27c6b86a6f7325524d788c852a9266f1f
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/606755
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Lasse Folger <lassefolger@google.com>
Reviewed-by: Christian Höppner <hoeppi@google.com>

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

@@ -163,27 +163,6 @@ func genReflectFileDescriptor(gen *protogen.Plugin, g *protogen.GeneratedFile, f
 	}
 
 	if len(f.allMessages) > 0 {
-		// Populate MessageInfo.Exporters.
-		g.P("if !", protoimplPackage.Ident("UnsafeEnabled"), " {")
-		for _, message := range f.allMessages {
-			if sf := f.allMessageFieldsByPtr[message]; len(sf.unexported) > 0 {
-				idx := f.allMessagesByPtr[message]
-				typesVar := messageTypesVarName(f)
-
-				g.P(typesVar, "[", idx, "].Exporter = func(v any, i int) any {")
-				g.P("switch v := v.(*", message.GoIdent, "); i {")
-				for i := 0; i < sf.count; i++ {
-					if name := sf.unexported[i]; name != "" {
-						g.P("case ", i, ": return &v.", name)
-					}
-				}
-				g.P("default: return nil")
-				g.P("}")
-				g.P("}")
-			}
-		}
-		g.P("}")
-
 		// Populate MessageInfo.OneofWrappers.
 		for _, message := range f.allMessages {
 			if len(message.Oneofs) > 0 {