sched_ext: Remove sched_class->switch_class()

With sched_ext converted to use put_prev_task() for class switch detection,
there's no user of switch_class() left. Drop it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>

kernel/sched/core.c

@@ -5965,22 +5965,12 @@ __pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 	for_each_active_class(class) {
 		if (class->pick_next_task) {
 			p = class->pick_next_task(rq, prev);
-			if (p) {
-				const struct sched_class *prev_class = prev->sched_class;
-
-				if (class != prev_class && prev_class->switch_class)
-					prev_class->switch_class(rq, p);
+			if (p)
 				return p;
-			}
 		} else {
 			p = class->pick_task(rq);
 			if (p) {
-				const struct sched_class *prev_class = prev->sched_class;
-
 				put_prev_set_next_task(rq, prev, p);
-
-				if (class != prev_class && prev_class->switch_class)
-					prev_class->switch_class(rq, p);
 				return p;
 			}
 		}

kernel/sched/sched.h

@@ -2385,8 +2385,6 @@ struct sched_class {
 	void (*put_prev_task)(struct rq *rq, struct task_struct *p, struct task_struct *next);
 	void (*set_next_task)(struct rq *rq, struct task_struct *p, bool first);
 
-	void (*switch_class)(struct rq *rq, struct task_struct *next);
-
 #ifdef CONFIG_SMP
 	int  (*select_task_rq)(struct task_struct *p, int task_cpu, int flags);