[heap] Remove obsolete left trimming code + comments.

Follow-up to r54219 ( https://chromium-review.googlesource.com/1124450 )

They're relevant if we also iterate the heap and check there are no pointers to
the original FixedArrayBase, but in the landed version of that CL we don't do
that.

BUG=chromium:859809

Change-Id: Iffd8b76e74b6690cde961d4c542cb16ddd934f33
Reviewed-on: https://chromium-review.googlesource.com/1131123Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54396}

src/elements.cc

@@ -2280,9 +2280,6 @@ class FastElementsAccessor : public ElementsAccessorBase<Subclass, KindTraits> {
     Handle<BackingStore> dst_elms = Handle<BackingStore>::cast(backing_store);
     if (len > JSArray::kMaxCopyElements && dst_index == 0 &&
         heap->CanMoveObjectStart(*dst_elms)) {
-      // Remove all the pointers to the FixedArrayBase we're going to left trim
-      // from the heap.
-      receiver->set_elements(ReadOnlyRoots(heap).empty_fixed_array());
       // Update all the copies of this backing_store handle.
       *dst_elms.location() =
           BackingStore::cast(heap->LeftTrimFixedArray(*dst_elms, src_index));