Don't explicitly require libxml2

Originally done by rjones as part of this package review:

  https://bugzilla.redhat.com/226055

libvirt.spec.in

@@ -200,7 +200,6 @@ the libvirtd server exporting the virtualization support.
 %package client
 Summary: client side library and utilities of the libvirt library
 Group: Development/Libraries
-Requires: libxml2
 Requires: readline
 Requires: ncurses
 # So remote clients can access libvirt over SSH tunnel