[cleanup] Remove deprecated code related to element by reference

Code was deprecated in Dune 2.6

dune/geometry/referenceelement.hh

@@ -22,10 +22,6 @@ namespace Dune {
     template<typename ctype, int dim>
     class ReferenceElementImplementation;
 
-    // forward declaration for deprecation check reference element type
-    template<typename ctype, int dim>
-    class DeprecatedReferenceElement;
-
     // forward declaration for backwards compatibility conversion
     template<typename ctype, int dim>
     struct ReferenceElements;
@@ -286,17 +282,6 @@ namespace Dune {
         return *_impl;
       }
 
-#ifndef DOXYGEN
-
-      DUNE_DEPRECATED_MSG("Capturing reference elements by reference is deprecated in DUNE 2.6. Please store a copy instead.")
-      operator const DeprecatedReferenceElement<ctype,dimension>&() const
-      {
-        return ReferenceElements<ctype,dimension>::deprecated(*this);
-      }
-
-#endif // DOXYGEN
-
-
       //! Compares for equality with another reference element.
       bool operator==(const ReferenceElement& r) const
       {