Revert "Engine: restore gamma when releasing display mode"

This reverts commit 188c042.

This is not a correct fix. The Gamma seems to be controlled well by SDL itself, but there are certain cases when it is not affected properly? This should be investigated further before any changes.

Engine/gfx/ali3dsw.cpp

@@ -238,12 +238,6 @@ void SDLRendererGraphicsDriver::ReleaseDisplayMode()
   OnModeReleased();
   ClearDrawLists();
 
-  SDL_Window *window = sys_get_window();
-  if (window && _hasGamma)
-  {
-      SDL_SetWindowGammaRamp(window, _defaultGammaRed, _defaultGammaGreen, _defaultGammaBlue);
-  }
-
   sys_window_set_style(kWnd_Windowed);
 }