Prior to GitLab 11.11, a pull mirror would only be shown if the project
had the `mirror` flag enabled.

After https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10361, a
pull mirror would be shown if import data were available. However,
projects can have import data but not be mirrors. Since import data only
gets deleted if the project were a mirror before, it became impossible
to remove a pull mirror from the interface.

This change reverts to the previous behavior for pull mirrors. Push
mirrors retain the same behavior as before.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/12871