block: Remove elevator required features

The only elevator feature ever implemented is ELEVATOR_F_ZBD_SEQ_WRITE
for signaling that a scheduler implements zone write locking to tightly
control the dispatching order of write operations to zoned block
devices. With the removal of zone write locking support in mq-deadline
and the reliance of all block device drivers on the block layer zone
write plugging to control ordering of write operations to zones, the
elevator feature ELEVATOR_F_ZBD_SEQ_WRITE is completely unused.
Remove it, and also remove the now unused code for filtering the
possible schedulers for a block device based on required features.

Signed-off-by: Damien Le Moal <[email protected]>
Reviewed-by: Hannes Reinecke <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Bart Van Assche <[email protected]>
Tested-by: Hans Holmberg <[email protected]>
Tested-by: Dennis Maisenbacher <[email protected]>
Reviewed-by: Martin K. Petersen <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

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