There are certain things we can leave in the database even when they
are not being used. The criteria for this is:
* Could it be used again in the future?
* Can it be excluded from queries easily while it is unused?
Examples are entries in the package table, and entries in the category table.
This fixes a problem where entries in the category-app join table stayed in
the database, causing categories to be considered as "in use" when really there
were no apps in those categories. These rows need to be removed, because when
new apps are added again in the future, they will have different primary keys.
These different primary keys mean that the rows in the category-app table will
never be useful again, and thus should be removed.
Fixes#806.
18885a66c2