btrfs: remove duplicated code to drop delayed ref during transaction abort

When destroying delayed refs during a transaction abort, we have open
coded the removal of a delayed ref, which is also done by the static
helper function drop_delayed_ref(). So remove that duplicated code and
use drop_delayed_ref() instead.

Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>