gnome.bbclass: remove to use oe-core's version

* suggested in [1]
* build tested from scratch with my working image [2]

[1] http://lists.linuxtogo.org/pipermail/openembedded-devel/2012-August/041157.html
[2] http://gitorious.org/schnitzeltony-oe-meta/meta-misc/blobs/master/recipes-image/xfce-full-image.bb



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