Unordered/hash: Avoid a gcc warning. Refs #6771

[SVN r77832]

test/exception/swap_exception_tests.cpp

@@ -3,8 +3,6 @@
 // Distributed under the Boost Software License, Version 1.0. (See accompanying
 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 
-#include "../helpers/prefix.hpp"
-
 #include "./containers.hpp"
 #include "../helpers/random_values.hpp"
 #include "../helpers/invariants.hpp"