Do not disable X11 ARGB visuals (fixes: #3581)

The XVideo and X11 outputs can cope with this properly now
(cherry picked from commit 9e47aa72)

bin/vlc.c

@@ -80,10 +80,6 @@ int main( int i_argc, const char *ppsz_argv[] )
     setenv ("GNOME_DISABLE_CRASH_DIALOG", "1", 1);
 # endif
 
-    /* Make Xlib hide visuals with an alphachannel. Ensure that Qt4 will not
-     * use the alpha channel for the embedded video window. */
-    setenv ("XLIB_SKIP_ARGB_VISUALS", "1", 1);
-
     /* Clear the X.Org startup notification ID. Otherwise the UI might try to
      * change the environment while the process is multi-threaded. That could
      * crash. Screw you X.Org. Next time write a thread-safe specification. */