ARM: smp: Drop RCU_NONIDLE usage in cpu_die()

Before f7b861b7 ("arm: Use generic idle loop") ARM would kill the
CPU within the rcu idle section.  Now that the rcu_idle_enter()/exit()
pair have been pushed lower down in the idle loop this is no longer true
and so using RCU_NONIDLE here is no longer necessary and also harmful
because RCU is not actually idle at this point.

Cc: Russell King <linux@arm.linux.org.uk>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>