gitlab-users-cleanup: Make sure to list all events

Otherwise, one may get the following:

    UserWarning: Calling a `list()` method without specifying `all=True`
    or `as_list=False` will return a maximum of 100 items. Your query
    returned 100 of 356 items. See
    https://python-gitlab.readthedocs.io/en/v3.5.0/api-usage.html#pagination
    for more details. If this was done intentionally, then this warning can
    be supressed by adding the argument `all=False` to the `list()` call.
    (python-gitlab: /usr/lib/python3/dist-packages/gitlab/client.py:879)