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>

include/linux/rcupdate.h

@@ -360,8 +360,6 @@ void rcu_user_exit(void);
 #else
 static inline void rcu_user_enter(void) { }
 static inline void rcu_user_exit(void) { }
-static inline void rcu_user_hooks_switch(struct task_struct *prev,
-					 struct task_struct *next) { }
 #endif /* CONFIG_NO_HZ_FULL */
 
 #ifdef CONFIG_RCU_NOCB_CPU