Don't initialize `Repo` in `external_repo`

[pared]

Here we are intializing repo just to get access to config.
Consider whether we could not just do that using Config and providing proper path.
Problem:
There is already a test that is requiring raising UrlNotDvcRepoError which is raised on Repo initialization.

[pared]

Kudos @Suor for pointing it out.

[pared]

After refactoring of external repo in #3190 I believe that this issue is obsolete. Closing.