Do not duplicate relative mouse mode state

The relative mouse mode is tracked by SDL, and accessible via
SDL_GetRelativeMouseMode().

This is more robust in case SDL changes the relative mouse mode on its
own.

app/src/usb/screen_otg.h

@@ -18,7 +18,6 @@ struct sc_screen_otg {
     SDL_Texture *texture;
 
     // See equivalent mechanism in screen.h
-    bool mouse_captured;
     SDL_Keycode mouse_capture_key_pressed;
 };