Filter out singleusergroup results from suggestions

If the singleusergroup plugin is installed alongside the reviewers
plugin, the reviewers plugin will suggest 'user/' groups from the
singleusergroup plugin despite not supporting them.

This change naively filters out 'user/' groups as the plugins are not
aware of each other.

Change-Id: I9bab8ea7db18d113b9f9a272b1ddef2ff4e0dd32