btrfs: remove trans parameter of merge_ref

Now that drop_delayed_ref() doesn't get the btrfs_trans_handle passed in
anymore, we can get rid of it in merge_ref() as well.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>