Remove type traits after migration to dune-common

Naturally, if anyone used Dune::Solvers::IsNumber, that would now be
Dune::IsNumber. But I believe nobody was using this anyway.

dune/solvers/common/CMakeLists.txt

@@ -13,5 +13,4 @@ install(FILES
     resize.hh
     staticmatrixtools.hh
     tuplevector.hh
-    typetraits.hh
     DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/dune/solvers/common)

dune/solvers/common/typetraits.hh

-#ifndef DUNE_SOLVERS_COMMON_TYPETRAITS_HH
-#define DUNE_SOLVERS_COMMON_TYPETRAITS_HH
-
-#include <type_traits>
-
-namespace Dune {
-namespace Solvers {
-  template <typename T>
-  struct IsNumber
-      : public std::integral_constant<bool, std::is_arithmetic<T>::value> {};
-
-  template <typename T>
-  struct IsNumber<std::complex<T>>
-      : public std::integral_constant<bool, IsNumber<T>::value> {};
-}
-}
-
-#endif