database: Remove more uses of Mocks.ExternalServices
Created by: ryanslade
Removed all uses outside of the database package. Unexported it. Removed all the mock fields apart from List as they weren't being used.
The final left over usages can be removed in another PR and are a bit tricker since they are called from within another method on the ExternalServiceStore interface.
Part of https://github.com/sourcegraph/sourcegraph/issues/32380