[crankshaft] Remove the useless HMapEnumLength instruction.

src/crankshaft/arm64/lithium-arm64.cc

Index: src/crankshaft/arm64/lithium-arm64.cc
diff --git a/src/crankshaft/arm64/lithium-arm64.cc b/src/crankshaft/arm64/lithium-arm64.cc
index 7a5bd993a3d9d00fd9a472056fba5d898463e14e..95ec5e08c198adb5ad627904de35f941eaefcc84 100644
--- a/src/crankshaft/arm64/lithium-arm64.cc
+++ b/src/crankshaft/arm64/lithium-arm64.cc
@@ -1743,12 +1743,6 @@ LInstruction* LChunkBuilder::DoLoadRoot(HLoadRoot* instr) {
 }
 
 
-LInstruction* LChunkBuilder::DoMapEnumLength(HMapEnumLength* instr) {
-  LOperand* map = UseRegisterAtStart(instr->value());
-  return DefineAsRegister(new(zone()) LMapEnumLength(map));
-}
-
-
 LInstruction* LChunkBuilder::DoFlooringDivByPowerOf2I(HMathFloorOfDiv* instr) {
   DCHECK(instr->representation().IsInteger32());
   DCHECK(instr->left()->representation().Equals(instr->representation()));