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/screen.h

@@ -60,7 +60,6 @@ struct sc_screen {
 
     bool event_failed; // in case SDL_PushEvent() returned an error
 
-    bool mouse_captured; // only relevant in relative mouse mode
     // To enable/disable mouse capture, a mouse capture key (LALT, LGUI or
     // RGUI) must be pressed. This variable tracks the pressed capture key.
     SDL_Keycode mouse_capture_key_pressed;