Documentation: drop as block elevator reference in switching-sched.txt

Remove 'as' for as is no longer supported, and we can not use
'elevator=as' any more.

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>