Only show pull mirrors if there is actually a mirror enabled
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.
Edited by Stan Hu