[turbofan] Restore basic write barrier elimination.

src/compiler/machine-operator.h

Index: src/compiler/machine-operator.h
diff --git a/src/compiler/machine-operator.h b/src/compiler/machine-operator.h
index faa3a5d4b47378788b54269bc731d52a23454c42..99d1b955c197566d9c3876b064fb34fe72029810 100644
--- a/src/compiler/machine-operator.h
+++ b/src/compiler/machine-operator.h
@@ -32,16 +32,6 @@ class OptionalOperator final {
   const Operator* const op_;
 };
 
-// Supported write barrier modes.
-enum WriteBarrierKind {
-  kNoWriteBarrier,
-  kMapWriteBarrier,
-  kPointerWriteBarrier,
-  kFullWriteBarrier
-};
-
-std::ostream& operator<<(std::ostream& os, WriteBarrierKind);
-
 
 // A Load needs a MachineType.
 typedef MachineType LoadRepresentation;