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/ast/smt_terms.h

@@ -6,7 +6,6 @@
 #include <util/irep.h>
 
 #include <solvers/smt2_incremental/response_or_error.h>
-#include <solvers/smt2_incremental/type_traits.h>
 
 #include "smt_index.h"
 #include "smt_sorts.h"
@@ -150,7 +149,7 @@ 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
   {
   };