Host: Remove unnecessary function

Remove Host_UpdateMainFrame(). The only non-empty call happened in
DolphinNoGUI which called s_update_main_frame_event.Set(), but
DolphinNoGUI never waits on that event.

Source/Core/DolphinQt/Host.cpp

@@ -280,9 +280,6 @@ void Host_PPCBreakpointsChanged()
 // We ignore these, and their purpose should be questioned individually.
 // In particular, RequestRenderWindowSize, RequestFullscreen, and
 // UpdateMainFrame should almost certainly be removed.
-void Host_UpdateMainFrame()
-{
-}
 
 void Host_RequestRenderWindowSize(int w, int h)
 {