package/mesa3d: Remove dependency for the libxml2 module of host-python

Remove the ugly BR2_PACKAGE_HOST_LIBXML2_PYTHON hack as well as
mesa3d was the only package needing it, this is not the case
anymore since:

http://cgit.freedesktop.org/mesa/mesa/commit/configure.ac?id=2de70fe23f320ce4f559e37057fe07b7af99cf5a



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>