libxml2: remove LSB-specific overrides

libxml2: remove LSB-specific overrides

The linuxstdbase override was used by the poky-lsb distribution, which
aimed to be compatible with the Linux Standard Base specification.

LSB is long-dead and poky-lsb was removed back in 2018 with the thud
release[1].

Clarify the recipe by deleting these obsolete and unused assignments.

[1] meta-yocto 00fdefff55 ("poky-altcfg: Replace poky-lsb with poky-altcfg")

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>