Removing token in external service does not remove associated repositories

Created by: attfarhan

• Sourcegraph version: Insiders

Steps to reproduce:

1. Add an external service, include a GitHub token
2. Go to the repositories page, see the list of repos associated with the token.
3. Enable a private repo (e.g. sourcegraph/infrastructure)
4. Edit the external service you added, and remove the token field.
5. Go back to the repositories page and notice all the repositories are still listed there.
6. Go to the private repo you enabled previously and notice that you can still access it.

Expected behavior:

I would expect not to be able to access a private repo associated with the token that was removed, and also not see all the repositories listed in the repositories page that were associated with the old token.

Actual behavior:

Can access any enabled repos associated with the old token, and can see all repos associated with the old token in the repositories list.