wrong logic in assertValidRoleAssignmentListResponse method

According to current logic, we always use the last of entities to
compare with ref. And, actually, we provide another method
assertRoleAssignmentInListResponse to judge whether the ref in the
list. So, the ref parameter could be removed.

Change-Id: I3f8786baa6964c0f17add2d669aa736732698ea7
Merge "wrong logic in assertValidRoleAssignmentListResponse method"