exo: remove gnome-mount wrapper

exo-mount was removed long time ago [1]

[1] http://git.xfce.org/xfce/exo/commit/?id=5d698bf98b190d7c76a6b406bd5f542b915d0bd1



Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>