VideoBackend: Remove PeekMessages method

The video thread and backend no longer create any windows, therefore
there will never be any messages dispatched to their thread.

Source/Core/VideoBackends/OGL/main.cpp

@@ -58,12 +58,6 @@ Make AA apply instantly during gameplay if possible
 
 namespace OGL
 {
-// Draw messages on top of the screen
-unsigned int VideoBackend::PeekMessages()
-{
-  return GLInterface->PeekMessages();
-}
-
 std::string VideoBackend::GetName() const
 {
   return "OGL";