rcu: Conditionally build CPU-hotplug teardown callbacks

Among the three CPU-hotplug teardown RCU callbacks, two of them early
exit if CONFIG_HOTPLUG_CPU=n, and one is left unchanged. In any case
all of them have an implementation when CONFIG_HOTPLUG_CPU=n.

Align instead with the common way to deal with CPU-hotplug teardown
callbacks and provide a proper stub when they are not supported.

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>