Fix missing cases of empty_string canonicalization

src/mips64/macro-assembler-mips64.h

Index: src/mips64/macro-assembler-mips64.h
diff --git a/src/mips64/macro-assembler-mips64.h b/src/mips64/macro-assembler-mips64.h
index 7693dfad5348e3d11b1fb2b0d980932a635d4b70..43916f8ef570276af2c5370950019f0dd69af799 100644
--- a/src/mips64/macro-assembler-mips64.h
+++ b/src/mips64/macro-assembler-mips64.h
@@ -1878,12 +1878,6 @@ const Operand& rt = Operand(zero_reg), BranchDelaySlot bd = PROTECT
                       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,  // ne for new space, eq otherwise.