xen/events: remove unlikely() from WARN() condition

"unlikely(WARN(x))" is excessive. WARN() already uses unlikely()
internally.

Link: http://lkml.kernel.org/r/20190829165025.15750-4-efremov@linux.com


Signed-off-by: Denis Efremov <efremov@linux.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Joe Perches <joe@perches.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>