Remove gpu_refresh_rate configuration option

Changing it makes emulation inherently inaccurate. It also had a wrong
default value (30, whereas the real system has a refresh rate of 60 Hz)
which, even if changed, would continue to be used unless people manually
removed it from their config files.

src/core/settings.h

@@ -35,7 +35,6 @@ struct Values {
     int pad_cright_key;
 
     // Core
-    int gpu_refresh_rate;
     int frame_skip;
 
     // Data Storage