sched: Fix a stale comment in pick_next_task()

fair_sched_class->next no longer exists since commit:

  a87e749e

 ("sched: Remove struct sched_class::next field").

Now the sched_class order is specified by the linker script.

Rewrite the comment in a more generic way.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210519063709.323162-1-masahiroy@kernel.org