Fix missing cases of empty_string canonicalization

src/s390/macro-assembler-s390.h

Index: src/s390/macro-assembler-s390.h
diff --git a/src/s390/macro-assembler-s390.h b/src/s390/macro-assembler-s390.h
index 91ef0c3eef01d112d6b840b48b56d3f6edb83680..dba1c938cb09702e261a4f5e937481141edbe710 100644
--- a/src/s390/macro-assembler-s390.h
+++ b/src/s390/macro-assembler-s390.h
@@ -1787,10 +1787,6 @@ class MacroAssembler : public Assembler {
                       bool* definitely_mismatches, InvokeFlag flag,
                       const CallWrapper& call_wrapper);
 
-  void InitializeNewString(Register string, Register length,
-                           Heap::RootListIndex map_index, Register scratch1,
-                           Register scratch2);
-
   // Helper for implementing JumpIfNotInNewSpace and JumpIfInNewSpace.
   void InNewSpace(Register object, Register scratch,
                   Condition cond,  // eq for new space, ne otherwise.