libxml2: upgrade 2.9.14 -> 2.10.3

Change ptest away from using the upstream Makefiles to manually running
the tests: they're not actually integrated with automake anyway so this
didn't gain us anything apart from patches we can't send upstream. Drops
the following patches:
- 0001-Make-ptest-run-the-python-tests-if-python-is-enabled.patch
- 0001-Port-gentest.py-to-Python-3.patch
- fix-execution-of-ptests.patch
- remove-fuzz-from-ptests.patch
- runtest.patch

Add a simple patch to install the test binaries via the Makefile:
- install-tests.patch

The Python module is built differently and a number of patches no longer
apply and appear to be redundant, remove:
- python-sitepackages-dir.patch

libxml-m4-use-pkgconfig.patch has been sent upstream now, mark as
backport.

Remove obsolete --without-docbook option.

Remove obsolete xml2Conf.sh packaging.

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