[runtime] Remove out-dated TODO

Bug: 
Change-Id: I0415b3946e6efd97c3b2fb770fda7dba265ee8cd
Reviewed-on: https://chromium-review.googlesource.com/545000Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46177}

src/elements.cc

@@ -1415,7 +1415,6 @@ class DictionaryElementsAccessor
 
 
   static void DeleteImpl(Handle<JSObject> obj, uint32_t entry) {
-    // TODO(verwaest): Remove reliance on index in Shrink.
     Handle<SeededNumberDictionary> dict(
         SeededNumberDictionary::cast(obj->elements()));
     dict = SeededNumberDictionary::DeleteEntry(dict, entry);