[turbofan] Remove a dead declaration

Change-Id: Iaa3abd6584adf6c844d09a6341bd7fb80fb3d24d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1932372Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Auto-Submit: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65149}

src/compiler/simplified-operator-reducer.h

@@ -37,8 +37,6 @@ class V8_EXPORT_PRIVATE SimplifiedOperatorReducer final
   Reduction Reduce(Node* node) final;
 
  private:
-  Reduction ReduceReferenceEqual(Node* node);
-
   Reduction Change(Node* node, const Operator* op, Node* a);
   Reduction ReplaceBoolean(bool value);
   Reduction ReplaceFloat64(double value);