extsvc/github: migrate mocks to vcr
Created by: davejrt
Removes no longer required mock tests in favour of VCR as part of https://github.com/sourcegraph/sourcegraph/issues/33839
- removes the conditional for mocks in GetRepository as this now uses vcr exclusively
- migrates TestClient_ListRepositoriesForSearch to vcr
- removes TestClient_ListOrgRepositories as this seems to be covered by TestListOrgRepositories
Test plan
A green build on this PR