icinga - fix dependency cycle for system user

err: Could not apply complete catalog: Found 1 dependency cycle:

(Group[icinga] => Generic::Systemuser[icinga] => Group[icinga])

groups are already a native dependency for the user, so this is not
needed and needs to be removed since we switched
to generic::systemuser

Change-Id: Ic81c2b99bdf72bef5f85569181279087b1444bdd