rcu: Update rcu_bind_gp_kthread() header comment

The header comment for rcu_bind_gp_kthread() refers to sysidle, which
is no longer with us.  However, it is still important to bind RCU's
grace-period kthreads to the housekeeping CPU(s), so rather than remove
rcu_bind_gp_kthread(), this commit updates the comment.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>

kernel/rcu/tree_plugin.h

@@ -2622,8 +2622,7 @@ static bool rcu_nohz_full_cpu(struct rcu_state *rsp)
 }
 
 /*
- * Bind the grace-period kthread for the sysidle flavor of RCU to the
- * timekeeping CPU.
+ * Bind the RCU grace-period kthreads to the housekeeping CPU.
  */
 static void rcu_bind_gp_kthread(void)
 {