[NFC] Remove redundant includes for <string>.

Author: varungandhi-apple

include/swift/IDE/SourceEntityWalker.h

@@ -17,7 +17,6 @@
 #include "swift/Basic/LLVM.h"
 #include "swift/Basic/SourceLoc.h"
 #include "llvm/ADT/PointerUnion.h"
-#include <string>
 
 namespace clang {
   class Module;

include/swift/SIL/SILWitnessTable.h

@@ -28,7 +28,6 @@
 #include "swift/AST/ProtocolConformanceRef.h"
 #include "llvm/ADT/ilist_node.h"
 #include "llvm/ADT/ilist.h"
-#include <string>
 
 namespace swift {