block: Remove unused definitions

The ELV_MQUEUE_XXX definitions in include/linux/elevator.h are unused
since the removal of elevator_may_queue_fn in kernel 5.0. Remove these
definitions and also remove the documentation of elevator_may_queue_fn
in Documentiation/block/biodoc.txt.

Acked-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>