libarchive: disable redundant libxml2 PACKAGECONFIG

An XML parser (either libxml2, or expat) is only needed by libarchive to
read/write XAR archives.  However, these also need OpenSSL enabled which
by default it is not, so XAR files were disabled and libxml2 was a build
dependency for no reason.

As XAR archives are mainly used on macOS, we can remove libxml2.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>