Do not specialize IsNumber for adouble

That's already done in adolcnamespaceinjections.hh, in dune-fufem.

test/adolctest.cc

@@ -27,15 +27,6 @@ typedef double FDType;
 #include <dune/fufem/utilities/adolcnamespaceinjections.hh>
 
 #include <dune/common/typetraits.hh>
-// This specialization is needed (in particular) to make copying of FieldMatrices work.
-namespace Dune {
-  template <>
-  struct IsNumber<adouble>
-  {
-    constexpr static bool value = true;
-  };
-}
-
 #include <dune/common/fmatrix.hh>
 
 #include <dune/geometry/quadraturerules.hh>