x86/xen: Remove unnecessary BUG_ON(preemptible()) in xen_setup_timer()

There is no reason for having it and, with commit 250a1ac6 ("x86,
smpboot: Remove pointless preempt_disable() in
native_smp_prepare_cpus()"), it prevents HVM guests from booting.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>