btrfs: raid56: remove unused BTRFS_RBIO_REBUILD_MISSING

Commit aca43fe8 ("btrfs: remove unused raid56 functions which were
dedicated for scrub") removed the special handling of RAID56 scrub for
missing device.

As scrub goes full mirror_num based recovery, that means if it hits a
missing device in RAID56, it would just try the next mirror, which would
go through the BTRFS_RBIO_READ_REBUILD operation.

This means there is no longer any use of BTRFS_RBIO_REBUILD_MISSING
operation and we can safely remove it.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>