Documentation:kernel-per-CPU-kthreads.txt: Remove reference to elevator=

This argument was not being considered since blk-mq was set by default,
so removed this documentation to avoid confusion.

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

.txt file is now .rst

Signed-off-by: Jens Axboe <axboe@kernel.dk>