Remove limit and offset manipulation when getting users or groups, because it does not work when more than one user or group backend. Fixing it would be too costly performancewise, so we switch back to the model used in OC 5: limit and offset are effective per backend, and not a general constraint
adjust tests accordingly
Merge pull request #8216 from owncloud/usermanager_fix_limitoffset

 	Remove limit and offset manipulation when getting users or groups, be…