[tidy] Remove stray iostream include from test.

It is unused and we cannot rely on standard headers being present while
executing tests.

llvm-svn: 251499

clang-tools-extra/test/clang-tidy/modernize-redundant-void-arg.cpp

@@ -1,6 +1,6 @@
 // RUN: %check_clang_tidy %s modernize-redundant-void-arg %t
 
-#include <iostream>
+#define NULL 0
 
 int foo();