store: remove dependency on database.DB
Created by: keegancsmith
We recently introduced the need to have a database.DB to speak to gitserver. In that PR (https://github.com/sourcegraph/sourcegraph/pull/32769) we threaded through the DB from searcher. However, we did not need to do that since we already have the DB via the closure.
On a higher level: The intention of the FetchTar function is to inject the minimal interface we need to describe what we want. DB does not change across requests, so we can remove it.
Note: There is also FilterTar which in theory doesn't need DB as the input. However, the way the function is currently defined it does since it directly uses the gitserver client.
Test Plan: CI