Remove gtk-doc documentations from target

On my target rootfs it is installed documentation for:
atk, cairo, gdk, gdk-pixbuf, gio, glib, gobject, gtk, pango

It ocupies 34M of space. Configure option --disable-gtk-doc doesn't
prevent installation of it. So we should remove them manually if
documentation is disabled.

[Peter: also remove manual rm's from hal/libxml2/libglade]
Signed-off-by: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>