python-pyxml: remove obsolete package

PyXML is bitrotten and obsolete. You'll find the functionality it
previously provided is now included in the Python standard library.

So no need to keep this package in Buildroot anymore.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>