Finish disentangling dbcore from beets globals

There are three places left in `beets.dbcore` that reference `beets.config` (find them with `ack config beets/dbcore`). The `dbcore` component is supposed to be independent of the rest of beets, so we need to remove these.

Run `ack config beets/dbcore` and inspect the three references in `beets/dbcore`. Trace what each use of `beets.config` provides before removing the dependency, then run the relevant project tests to confirm dbcore still works independently of beets globals.

Written by the indexing model from the issue text.