remove_relationship
Description:
Removes a relationship of the specified type with the specified target.
Parameters:
| this | 
           The Object from which an AtkRelation is to be removed.  | 
      
| relationship | 
           The RelationType of the relation  | 
      
| target | 
           The Object which is the target of the relation to be removed.  | 
      
Returns:
| 
           TRUE if the relationship is removed.  |