use `... = delete;` instead of just private functions

dune/common/bitsetvector.hh

@@ -198,7 +198,7 @@ namespace Dune {
        This is only a Proxy class, you can't get the address of the
        object it references
      */
-    void operator & ();
+    void operator & () = delete;
 
     friend class BitSetVectorReference<block_size, Alloc>;
   };

dune/common/parallel/remoteindices.hh

@@ -402,8 +402,7 @@ namespace Dune {
 
   private:
     /** copying is forbidden. */
-    RemoteIndices(const RemoteIndices&)
-    {}
+    RemoteIndices(const RemoteIndices&) = delete;
 
     /** @brief Index set used at the source of the communication. */
     const ParallelIndexSet* source_;