When destroying a project, cleanup the .hg folders too

Problem

When a project is destroyed, Gitlab removes the .git folders (one for the wiki, one for the repository), but leaves the corresponding .hg folders.

Besides occupying useless space, the behavior is unknown if a new project with the same name is created. It may either crash at the creation of the folder, or be populated with the previous contents instead of being empty.

Task description

Remove the .hg folders along with the .git ones.