Btrfs: remove no longer used 'sync' member from transaction handle

Commit db2462a6 ("btrfs: don't run delayed refs in the end transaction
logic") removed its last use, so now it does absolutely nothing, therefore
remove it.

Reviewed-by: Nikolay Borisov <nborisov@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>