btrfs: remove no longer used trans_list member of struct btrfs_ordered_extent

The 'trans_list' member of an ordered extent was used to keep track of the
ordered extents for which a transaction commit had to wait. These were
ordered extents that were started and logged by an fsync. However we don't
do that anymore and before we stopped doing it we changed the approach to
wait for the ordered extents in commit 161c3549 ("Btrfs: change how
we wait for pending ordered extents"), which stopped using that list and
therefore the 'trans_list' member is not used anymore since that commit.
So just remove it since it's doing nothing and making each ordered extent
structure waste memory (2 pointers).

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>