Fix compound unique index for hierarchies_table

[fkmy]

related issue: #410

I feel that the unique compound index of ancestor_id and descendant_id, is needed and generations is redundant.
Because there should be only one kind of generations for a set of ancestor_id and descendant_id.

The current table definition allows for the creation of multiple depth parent-child relationships.

example

items

id	content	parent_id
1	"Parent"	NULL
2	"First Child"	1
3	"Second Child"	1
4	"First Grandchild"	3

item_hierarchies

ancestor_id	descendant_id	generations
1	1	0
1	2	1
2	2	0
1	3	1
3	3	0
1	4	2
3	4	1
4	4	0

Before the refurbishment, this record could also be added to the above.

INSERT INTO `item_hierarchies` (`ancestor_id`, `descendant_id`, `generations`) VALUES (1, 4, 1);

However, If there is a search by ancestor_id, descendant_id and generations columns, considering performance point of view, the disappearance of the ancestor_id, descendant_id and generations composite indexes is not good.
Therefore, I think it needs to be redefined.