rcu: Remove unused stop-machine #include

Long ago, RCU used the stop-machine mechanism to implement expedited
grace periods, but no longer does so.  This commit therefore removes
the no-longer-needed #includes of linux/stop_machine.h.

Link: https://lwn.net/Articles/805317/Reported-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree.c

@@ -43,7 +43,6 @@
 #include <uapi/linux/sched/types.h>
 #include <linux/prefetch.h>
 #include <linux/delay.h>
-#include <linux/stop_machine.h>
 #include <linux/random.h>
 #include <linux/trace_events.h>
 #include <linux/suspend.h>

kernel/rcu/tree.h

@@ -16,7 +16,6 @@
 #include <linux/cpumask.h>
 #include <linux/seqlock.h>
 #include <linux/swait.h>
-#include <linux/stop_machine.h>
 #include <linux/rcu_node_tree.h>
 
 #include "rcu_segcblist.h"