Fix missing cases of empty_string canonicalization

src/arm64/macro-assembler-arm64.h

Index: src/arm64/macro-assembler-arm64.h
diff --git a/src/arm64/macro-assembler-arm64.h b/src/arm64/macro-assembler-arm64.h
index 8b1c89b183123263936c70164440a11e264b75bc..68a23bf1fef5a735a69bbd800415856a2836c792 100644
--- a/src/arm64/macro-assembler-arm64.h
+++ b/src/arm64/macro-assembler-arm64.h
@@ -2003,12 +2003,6 @@ class MacroAssembler : public Assembler {
   CPURegList tmp_list_;
   CPURegList fptmp_list_;
 
-  void InitializeNewString(Register string,
-                           Register length,
-                           Heap::RootListIndex map_index,
-                           Register scratch1,
-                           Register scratch2);
-
  public:
   // Far branches resolving.
   //