use add-apt-repository instead of add-apt-repository

In Ubuntu Bionic 18.04 and Focal 20.04, the command remove-apt-repository
either does not exists or fail in a default installation with the error:

    ModuleNotFoundError: No module named 'softwareproperties'

But add-apt-repository allow removal through the `-r` flag.

cdist/conf/type/__apt_ppa/gencode-remote

@@ -32,6 +32,6 @@ case "$state_should" in
       echo "add-apt-repository -y '$name'"
    ;;
    absent)
-      echo "remove-apt-repository '$name'"
+      echo "add-apt-repository -r -y '$name'"
    ;;
 esac