repo-updater: load data solely from database in `RepoLookup`

Created by: mrnugget

This fixes https://github.com/sourcegraph/sourcegraph/issues/3466 by changing the code in RepoLookup to only look into the database for information about a repository.

Since https://github.com/sourcegraph/sourcegraph/pull/3884 and https://github.com/sourcegraph/sourcegraph/pull/3878 have been merged, this is what's been happening anyway, so this PR only removes "dead code".

In addition to that it also removes the "writing back to the frontend" functionality: https://github.com/sourcegraph/sourcegraph/blob/master/cmd/repo-updater/repoupdater/server.go#L401

What this does not include:

• Storing repo links in the repo.sources column and serving them straight from the database instead of constructing them on the fly here in RepoLookup -- I will tackle this in another PR/branch
• Taking into account that Sourcegraph.com needs a separate endpoint that does not serve information from the database but instead contacts the code hosts directly -- @keegancsmith offered to take care of this, but if that's not possible, let me know and I'll try to update this PR Update: this is included in the PR, but in a different form, see comments below

Test plan: go test