btrfs: drop unused parameter transaction from alloc_log_tree()

The function got split in commit 6ab6ebb7 ("btrfs: split
alloc_log_tree()") and since then transaction parameter has been unused.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>