Removed a header leftover from earlier draft

Author: Kevin Frei

llvm/lib/DebugInfo/DWARF/DWARFVerifier.cpp

@@ -9,7 +9,6 @@
 #include "llvm/ADT/IntervalMap.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallSet.h"
-#include "llvm/ADT/StringExtras.h"
 #include "llvm/BinaryFormat/Dwarf.h"
 #include "llvm/DebugInfo/DWARF/DWARFAbbreviationDeclaration.h"
 #include "llvm/DebugInfo/DWARF/DWARFAttribute.h"