drop libxml2 checks since libxml is not actually used (for parallels)

For a long time, we assumed that libxml2 is necessary for parallels
block format support (block/parallels*). However, this format actually
does not use libxml [*]. Since this is the only user of libxml2 in
whole QEMU tree, we can drop all libxml2 checks and dependencies too.

It is even more: --enable-parallels configure option was the only
option which was silently ignored when it's (fake) dependency
(libxml2) isn't installed.

Drop all mentions of libxml2.

[*] Actually the basis for libxml use were introduced in commit
    ed279a06c5 ("configure: add dependency") but the implementation
    was never merged:
    https://lore.kernel.org/qemu-devel/70227bbd-a517-70e9-714f-e6e0ec431be9@openvz.org/

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20220119090423.149315-1-mjt@msgid.tls.msk.ru>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
[PMD: Updated description and adapted to use lcitool]
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220121154134.315047-5-f4bug@amsat.org>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20220204204335.1689602-9-alex.bennee@linaro.org>

scripts/ci/org.centos/stream/8/x86_64/configure

@@ -81,7 +81,6 @@
 --disable-libssh \
 --disable-libudev \
 --disable-libusb \
---disable-libxml2 \
 --disable-linux-aio \
 --disable-linux-io-uring \
 --disable-linux-user \