Added rm to the existing dir to allow re:clone on scheduled mirror

The isis dir exists on multiple CI runs. We want a clean mirror, no matter what might happen upstream (e.g., a history re-write). Added an rm to blast the isis directory from the GitLab instance as the history on that repo could be out of date (e.g., if a history change is pushed to the ISIS GitHub repo and not the GitLab repo). This changes means only the GitHub repo contains the history of record for the project.