Remove local declaration of void_t

With the move to C++ 17 we can use std::void_t instead.

Author: tautschnig

src/solvers/smt2_incremental/smt_terms.h

@@ -7,7 +7,6 @@
 
 #include <solvers/smt2_incremental/smt_index.h>
 #include <solvers/smt2_incremental/smt_sorts.h>
-#include <solvers/smt2_incremental/type_traits.h>
 
 #include <functional>
 #include <utility>
@@ -148,7 +147,7 @@ class smt_function_application_termt : public smt_termt
   template <class functiont>
   struct has_indicest<
     functiont,
-    void_t<decltype(std::declval<functiont>().indices())>> : std::true_type
+    std::void_t<decltype(std::declval<functiont>().indices())>> : std::true_type
   {
   };