Remove header

Author: zeyi2

clang/lib/Tooling/Tooling.cpp

@@ -37,7 +37,6 @@
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/IntrusiveRefCntPtr.h"
 #include "llvm/ADT/StringRef.h"
-#include "llvm/ADT/StringSet.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Option/OptTable.h"