repo-updater: Do not specify a default username and dbname
Created by: keegancsmith
Rather rely on the logic psql does when connecting to it. This is what Sourcegraph did in 3.2 and before. During 3.2 we hardcoded sourcegraph as a default, which required us in devenvs to specify PGDATABASE. Note: For deploy-sourcegraph and server PGDATABASE was always specified.
Test plan: go test works without specifying PG* vars.
Fixes https://github.com/sourcegraph/sourcegraph/issues/3409