xwayland: Delete all frame_callback_list nodes in xwl_unrealize_window

We were only calling xwl_present_unrealize_window for the toplevel
window, but the list can contain entries from child windows as well,
in which case we were leaving dangling pointers to freed memory.

Closes: xorg/xserver#1000
Fixes: c5067fea "xwayland: Use single frame callback for Present
                     flips and normal updates"
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>
Tested-by: Olivier Fourdan <ofourdan@redhat.com>
4 jobs for xwl_unrealize_window-frame_callback_list in 5 minutes and 53 seconds