[stubs] Remove dead string copying code

src/x64/code-stubs-x64.h

Index: src/x64/code-stubs-x64.h
diff --git a/src/x64/code-stubs-x64.h b/src/x64/code-stubs-x64.h
index a1813772218520a69ab62b3f3901059991559fb3..9c19c02f45587500112f262df7fb5987802b0ee2 100644
--- a/src/x64/code-stubs-x64.h
+++ b/src/x64/code-stubs-x64.h
@@ -14,15 +14,6 @@ void ArrayNativeCode(MacroAssembler* masm, Label* call_generic_code);
 
 class StringHelper : public AllStatic {
  public:
-  // Generate code for copying characters using the rep movs instruction.
-  // Copies rcx characters from rsi to rdi. Copying of overlapping regions is
-  // not supported.
-  static void GenerateCopyCharacters(MacroAssembler* masm,
-                                     Register dest,
-                                     Register src,
-                                     Register count,
-                                     String::Encoding encoding);
-
   // Compares two flat one-byte strings and returns result in rax.
   static void GenerateCompareFlatOneByteStrings(
       MacroAssembler* masm, Register left, Register right, Register scratch1,