Documenation: switching-sched: Remove notes about elevator argument

This argument was ignored since blk-mq was set as default, so remove it
from documentation.

Reviewed-by: Hannes Reinecke <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Marcos Paulo de Souza <[e-mail]>

.txt file is now .rst

Signed-off-by: Jens Axboe <[e-mail]>

Documentation/block/switching-sched.rst

···
 Switching Scheduler
 ===================
 
-To choose IO schedulers at boot time, use the argument 'elevator=deadline'.
-'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
 in::