Fix flatpak build
    
    Remove a too large icon file that's outside of hicolor-icon-spec.
    
    error: /tmp/tmpry0af3tm/build/export/share/icons/hicolor/1024x1024/apps/net.supertuxkart.SuperTuxKart.png is not a valid icon: Image too large (1024x1024). Max. size 512x512