Remove unnecessary check to see if trustee exists
Remove unnecessary check to see if trustee exists

Previously we check if the returned trustee value is None, this
is already done at the backend level.

Change-Id: I801bed09300782e289ee505612df09f7270fcb4e

dims/keystone,JioCloud/keystone,cernops/keystone,himanshu-setia/keystone,MaheshIBM/keystone,idjaw/keystone,takeshineshiro/keystone,rodrigods/keystone,rushiagr/keystone,jonnary/keystone,mahak/keystone,rajalokan/keystone,ilay09/keystone,jamielennox/keystone,jonnary/keystone,dstanek/keystone,blueboxgroup/keystone,roopali8/keystone,himanshu-setia/keystone,UTSA-ICS/keystone-kerberos,vivekdhayaal/keystone,roopali8/keystone,promptworks/keystone,nuxeh/keystone,klmitch/keystone,rodrigods/keystone,MaheshIBM/keystone,promptworks/keystone,klmitch/keystone,blueboxgroup/keystone,ajayaa/keystone,ging/keystone,jumpstarter-io/keystone,UTSA-ICS/keystone-kerberos,reeshupatel/demo,cernops/keystone,vivekdhayaal/keystone,mahak/keystone,reeshupatel/demo,jumpstarter-io/keystone,dims/keystone,dstanek/keystone,rajalokan/keystone,rajalokan/keystone,ilay09/keystone,takeshineshiro/keystone,dstanek/keystone,idjaw/keystone,openstack/keystone,mahak/keystone,promptworks/keystone,JioCloud/keystone,jumpstarter-io/keystone,ging/keystone,nuxeh/keystone,maestro-hybrid-cloud/keystone,vivekdhayaal/keystone,jamielennox/keystone,openstack/keystone,ilay09/keystone,maestro-hybrid-cloud/keystone,rushiagr/keystone,openstack/keystone,nuxeh/keystone,ajayaa/keystone,rushiagr/keystone,reeshupatel/demo