btrfs: do not return errors from submit_bio_hook_t instances

Both btrfs_repair_one_sector and submit_bio_one as the direct caller of
one of the instances ignore errors as they expect the methods themselves
to call ->bi_end_io on error.  Remove the unused and dangerous return
value.

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