[cleanup] Remove undefined extension helpers

Bug: v8:10155

Change-Id: I0ce8a71ab364f9281ab39df1f057bd93d7bef061
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2033165
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Auto-Submit: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66067}

src/objects/contexts.h

@@ -679,8 +679,6 @@ class Context : public HeapObject {
 #endif
 
   OBJECT_CONSTRUCTORS(Context, HeapObject);
-  DECL_INT_ACCESSORS(length_and_extension_flag)
-  DECL_SYNCHRONIZED_INT_ACCESSORS(length_and_extension_flag)
 };
 
 class NativeContext : public Context {