SDL_sysvideo.h: remove SDL_DisplayModeData typedef:

Fixes a type redefinition error:

In file included from /tmp/SDL3/test/../src/events/SDL_events_c.h:28,
                 from /tmp/SDL3/test/../src/events/SDL_pen.c:26,
                 from /tmp/SDL3/test/testautomation_pen.c:83:
/tmp/SDL3/test/../src/events/../video/SDL_sysvideo.h:34: error: redefinition of typedef 'SDL_DisplayModeData'
/tmp/SDL3/include/SDL3/SDL_video.h:78: note: previous declaration of 'SDL_DisplayModeData' was here

src/video/SDL_sysvideo.h

@@ -31,7 +31,6 @@ typedef struct SDL_VideoDisplay SDL_VideoDisplay;
 typedef struct SDL_VideoDevice SDL_VideoDevice;
 typedef struct SDL_VideoData SDL_VideoData;
 typedef struct SDL_DisplayData SDL_DisplayData;
-typedef struct SDL_DisplayModeData SDL_DisplayModeData;
 typedef struct SDL_WindowData SDL_WindowData;
 
 typedef struct