xAODRootAccess: Remove DataBucketBase::clone().

DataBucketBase::clone() doesn't appear to be used anywhere.

Control/xAODRootAccess/Root/TEventProxyDict.cxx

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2018 CERN for the benefit of the ATLAS collaboration
 */
 
 // $Id: TEventProxyDict.cxx 796514 2017-02-10 04:33:07Z ssnyder $
@@ -113,12 +113,6 @@ namespace xAODPrivate {
          return const_cast< void* >( result );
       }
 
-      /// Clone the current bucket
-      DataBucketBase* clone() const override {
-
-         return new THolderBucket( m_key, m_ti, m_event );
-      }
-
       /// Give up ownership of the bucket's contents. A no-op.
       void relinquish() override {}
       /// Lock the held object. A no-op.