x11: Removed an assert.

This assert triggers when run under XMonad. It's safe to pass a zero here
anyhow, as this will still work "well enough" and the original
problem--GNOME printing a warning message--is still fixed because GNOME's
window manager gives us a chance to grab a non-zero user-time value before
this code is run.

src/video/x11/SDL_x11window.c

@@ -1047,7 +1047,6 @@ SetWindowActive(_THIS, SDL_Window * window)
     if (X11_IsWindowMapped(_this, window)) {
         XEvent e;
 
-        SDL_assert(data->user_time != 0);  /* should be set by _some_ event by now. */
         /*printf("SDL Window %p: sending _NET_ACTIVE_WINDOW with timestamp %lu\n", window, data->user_time);*/
 
         SDL_zero(e);