iomap: remove IOMAP_F_ZONE_APPEND

No users left now that btrfs takes REQ_OP_WRITE bios from iomap and
splits and converts them to REQ_OP_ZONE_APPEND internally.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>