Remove global ServerService variable

This variable didn't seem to be used anyway, so no further changes were
required.

cmd/frps/root.go

@@ -203,7 +203,6 @@ func runServer(cfg config.ServerCommonConf) (err error) {
 		return err
 	}
 	log.Info("Start frps success")
-	server.ServerService = svr
 	svr.Run()
 	return
 }

server/service.go

@@ -51,8 +51,6 @@ const (
 	connReadTimeout time.Duration = 10 * time.Second
 )
 
-var ServerService *Service
-
 // Server service
 type Service struct {
 	// Dispatch connections to different handlers listen on same port