use_group is only honored if it is defined.

The "real" permission check is reading the logfile - the group check
in aa-notify is just an annoying additional check, and the default
"admin" only works on Ubuntu (other distributions typically use
"wheel").

This commit comments out use_group in the default config, which allows
everybody to use aa-notify. Permissions for reading the log file are of
course still needed.

References: https://bugzilla.opensuse.org/show_bug.cgi?id=1058787