fix: prevent running multiple app instances on Linux (#1385)

linux/my_application.cc

@@ -99,6 +99,5 @@ static void my_application_init(MyApplication* self) {}
 MyApplication* my_application_new() {
   return MY_APPLICATION(g_object_new(my_application_get_type(),
                                      "application-id", APPLICATION_ID,
-                                     "flags", G_APPLICATION_NON_UNIQUE,
                                      nullptr));
 }