[compiler] Remove unused CellRef method

Bug: v8:7790
Change-Id: Ibe41dcc3d1717326b8ce7bf3491bf32a8d0882b0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2421810
Commit-Queue: Georg Neis <neis@chromium.org>
Auto-Submit: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70027}

src/compiler/heap-refs.h

@@ -891,8 +891,6 @@ class CellRef : public HeapObjectRef {
   DEFINE_REF_CONSTRUCTOR(Cell, HeapObjectRef)
 
   Handle<Cell> object() const;
-
-  ObjectRef value() const;
 };
 
 class JSGlobalObjectRef : public JSObjectRef {