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: NHannes Reinecke <hare@suse.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NMarcos Paulo de Souza <marcos.souza.org@gmail.com>

.txt file is now .rst
Signed-off-by: NJens Axboe <axboe@kernel.dk>

Documentation/block/switching-sched.rst

@@ -2,10 +2,6 @@
 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::