[turbofan] Remove dead code from simplified lowering.

src/compiler/simplified-lowering.h

Index: src/compiler/simplified-lowering.h
diff --git a/src/compiler/simplified-lowering.h b/src/compiler/simplified-lowering.h
index 4b9e86b786c3839f8b7f21437b9e43061e2a3d27..6b2790c20cb44c71f39076f66947ee02608e98cc 100644
--- a/src/compiler/simplified-lowering.h
+++ b/src/compiler/simplified-lowering.h
@@ -54,10 +54,6 @@ class SimplifiedLowering final {
   // position information via the SourcePositionWrapper like all other reducers.
   SourcePositionTable* source_positions_;
 
-  Node* SmiTag(Node* node);
-  Node* IsTagged(Node* node);
-  Node* Untag(Node* node);
-  Node* OffsetMinusTagConstant(int32_t offset);
   Node* ComputeIndex(const ElementAccess& access, Node* const key);
   Node* StringComparison(Node* node);
   Node* Int32Div(Node* const node);