Revert "xen/smp: Fix CPU online/offline bug triggering a BUG: scheduling while atomic."

This reverts commit 41bd956d.

The fix is incorrect and not appropiate for the latest kernels.
In fact it _causes_ the BUG: scheduling while atomic while
doing vCPU hotplug.
Suggested-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/smp.c

@@ -432,13 +432,6 @@ static void __cpuinit xen_play_dead(void) /* used only with HOTPLUG_CPU */
 	play_dead_common();
 	HYPERVISOR_vcpu_op(VCPUOP_down, smp_processor_id(), NULL);
 	cpu_bringup();
-	/*
-	 * Balance out the preempt calls - as we are running in cpu_idle
-	 * loop which has been called at bootup from cpu_bringup_and_idle.
-	 * The cpucpu_bringup_and_idle called cpu_bringup which made a
-	 * preempt_disable() So this preempt_enable will balance it out.
-	 */
-	preempt_enable();
 }
 
 #else /* !CONFIG_HOTPLUG_CPU */