Update git submodules
Update git submodules

* Update plugins/singleusergroup from branch 'stable-2.16'
  to 2a38a8413c25fec910fb843932aa869647bdc1e7
  - SingleUserGroupTest: Remove unnecessary groupBackend member
    
    The GroupBackend is provided in the base class.
    
    Change-Id: I61500178ca37af70e002f4d46fcdc534f8cde325
    
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