Remove now unused package name from apk table

The package name is only stored in the fdroid_app table now, so we need to remove it form the fdroid_apk table. Under normal circumstances, I'd normally just leave unused fields in the DDL (the SQL which defines the tables) and never use it from within the Java code. However in this case, the package name formed part of the primary key of this table. Seeing as we are not inserting into that column any more, it isn't okay to leave it there but instead it must be removed so that we can put a more appropriate primary key on the table. In this case, the new primary key is appId + vercode + repoId.

I think this is the final merge request before I submit a MR with repo priorities.