m4: Drop libxml2 version number from configure help

We don't include this information for any other library, and
having it there means there are two places we need to change
every time the required version is bumped.

configure will provide the user with a nice error message,
which includes the required version, if libxml2 found on the
system is too old.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>

m4/virt-libxml.m4

@@ -18,7 +18,7 @@ dnl <http://www.gnu.org/licenses/>.
 dnl
 
 AC_DEFUN([LIBVIRT_ARG_LIBXML], [
-  LIBVIRT_ARG_WITH([LIBXML], [libxml-2.0 (>= 2.9.1) location], [check])
+  LIBVIRT_ARG_WITH([LIBXML], [libxml-2.0 location], [check])
 ])
 
 AC_DEFUN([LIBVIRT_CHECK_LIBXML], [