rcu: Remove CPU-hotplug failsafe from force-quiescent-state code path

Now that quiescent states for newly offlined CPUs are reported either
when that CPU goes offline or at the end of grace-period initialization,
the CPU-hotplug failsafe in the force-quiescent-state code path is no
longer needed.

This commit therefore removes this failsafe.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>