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>

arch/x86/xen/time.c

@@ -458,8 +458,6 @@ void xen_setup_timer(int cpu)
 
 void xen_setup_cpu_clockevents(void)
 {
-	BUG_ON(preemptible());
-
 	clockevents_register_device(this_cpu_ptr(&xen_clock_events.evt));
 }