task-xfce-extended: remove xfce4-icon-theme

xfce4-icon-theme is not required by default since xfce4-settings initially
works with gnome-icon-theme and gnome-icon-theme is pulled in by many other
recipes anyway.
To reduce rootfs size - especially after gtk-update-icon-cache -
xfce4-icon-theme is removed.

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>