rcu: Remove rcu_user_hooks_switch

Because there are neither uses nor intended uses for the
rcu_user_hooks_switch() function that was orginally intended
for nohz use, this commit removes it.

Signed-off-by: Yang Shi <yang.shi@linaro.org>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>