xen: remove stray preempt_disable() from PV AP startup code

In cpu_bringup() there is a call of preempt_disable() without a paired
preempt_enable(). This is not needed as interrupts are off initially.
Additionally this will result in early boot messages like:

BUG: scheduling while atomic: swapper/1/0/0x00000002
Signed-off-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20210825113158.11716-1-jgross@suse.comSigned-off-by: Juergen Gross <jgross@suse.com>

arch/x86/xen/smp_pv.c

@@ -64,7 +64,6 @@ static void cpu_bringup(void)
 	cr4_init();
 	cpu_init();
 	touch_softlockup_watchdog();
-	preempt_disable();
 
 	/* PVH runs in ring 0 and allows us to do native syscalls. Yay! */
 	if (!xen_feature(XENFEAT_supervisor_mode_kernel)) {