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>

Documentation/block/switching-sched.txt

 To choose IO schedulers at boot time, use the argument 'elevator=deadline'.
-'noop', 'as' and 'cfq' (the default) are also available. IO schedulers are
-assigned globally at boot time only presently.
+'noop' and 'cfq' (the default) are also available. IO schedulers are assigned
+globally at boot time only presently.
 
 Each io queue has a set of io scheduler tunables associated with it. These
 tunables control how the io scheduler works. You can find these entries