Remove uneeded forceRoundTrip on XdgOutputManager

In the first rountrip we will get all wl_outputs and the
xdg_output_manager. Therefore we know we will have called wl_output.bind
and xdg_output_manager_get_output by the time we hit the
second roundtrip on startup.

Change-Id: I69f911c13f9bcdfb59b04eceea4bcca778e7755c
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

src/plugins/platforms/wayland/qwaylanddisplay.cpp

@@ -597,7 +597,6 @@ void QWaylandDisplay::registry_global(uint32_t id, const QString &interface, uin
         mXdgOutputManager.reset(new QWaylandXdgOutputManagerV1(this, id, version));
         for (auto *screen : std::as_const(mWaitingScreens))
             screen->initXdgOutput(xdgOutputManager());
-        forceRoundTrip();
     }
 
     mGlobals.append(RegistryGlobal(id, interface, version, registry));