rcu: Remove unused local variable "cpu"

One danger of using __maybe_unused is that the compiler doesn't yell
at you when you remove the last reference, witness rcu_bind_gp_kthread()
and its local variable "cpu".  This commit removes this local variable.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree_plugin.h

@@ -2670,8 +2670,6 @@ static bool rcu_nohz_full_cpu(struct rcu_state *rsp)
  */
 static void rcu_bind_gp_kthread(void)
 {
-	int __maybe_unused cpu;
-
 	if (!tick_nohz_full_enabled())
 		return;
 	housekeeping_affine(current, HK_FLAG_RCU);