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/Null/VideoBackend.h

@@ -16,7 +16,5 @@ class VideoBackend : public VideoBackendBase
   std::string GetName() const override { return "Null"; }
   std::string GetDisplayName() const override { return "Null"; }
   void InitBackendInfo() override;
-
-  unsigned int PeekMessages() override { return 0; }
 };
 }