libnftnl: drop useless BR2_PACKAGE_LIBNFTNL_XML

Commit fa0d1ebf (libnftnl: bump to version 1.0.9) removed the mxml
dependency. Remove the redundant BR2_PACKAGE_LIBNFTNL_XML config symbol
as well.

Cc: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>