Database pruning

On @raggi's recommendation, I started looking at the database schema for unused or unnecessary data. Below is a list of what I found. @indirect or @hone, can you confirm or deny their usefulness?

 - Entire `linksets` table
 - `created_at` and `updated_at` on all tables. Set on insert but never used in a query.
 - `dependencies.unresolved_name` and its index
 - `rubygems.downloads`
 - `versions.authors`
 - `versions.description`
 - `versions.built_at` and its index
 - `versions.position` and its index
 - `versions.latest`

### Deploy

- [ ] Drop `HEROKU_POSTGRESQL_IVORY` after migration is deployed and known working.