sched_ext: Remove switch_class_scx()

Now that put_prev_task_scx() is called with @next on task switches, there's
no reason to use sched_class.switch_class(). Rename switch_class_scx() to
switch_class() and call it from put_prev_task_scx().

Signed-off-by: Tejun Heo <tj@kernel.org>