Remove all user code paths to manually add repositories

Created by: nicksnyder

This addresses the first checkbox in https://github.com/sourcegraph/sourcegraph/issues/914#issuecomment-440067022

The only supported way to add repositories to Sourcegraph is by configuring external services.

This does not impact auto enablement on sourcegraph.com because that code path does not go through the graphql api: https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/cmd/frontend/backend/repos.go#L69:12