drop non functional gtk cursor theme

Since we're using Xserver's builtin cursor font, the whole code reacting
Gtk cursors theme signals isn't doing anything useful at all - it just
recreates the cursors from the same source they originally came from
and readjust them for all clients, practically doesn't change anything.

IOW: we haven't any cursor theming in xfwm at all, just many Loc that
don't do anything useful.

If we'll actually implement that feature, it should be quite different
in several places:

1. directly use XSETTINGS spec instead of proprietary Gtkisms
2. carefully consider extending the spec to to also store the actual cursors
   (instead of just the theme name - since remote client don't  necessarily
   have the same themes)
3. do it in ScreenInfo instead of DisplayInfo - since xsettings operates
   on root window, thus on Xscreen instead of Xdisplay.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>