Fix unused variable

Author: Dobiasd

test/show_versions.cpp

@@ -44,5 +44,5 @@ TEST_CASE("show_versions - std_thread")
 TEST_CASE("show_versions - std_async")
 {
     auto handle = std::async(std::launch::async, []() { return 1; });
-    auto result = handle.get();
+    handle.get();
 }