rcu: Remove comment referring to __stop_machine()

Although it used to be that CPU_DYING notifiers executed on the outgoing
CPU with interrupts disabled and with all other CPUs spinning, this is
no longer the case.  This commit therefore removes this obsolete comment.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>