[ios] Remove delegate reset in OverlayPresenterImpl::OverlayWasDismissed

OverlayPresenterImpl's |detached_presenting_request_queue_| is
used to receive the OverlayRequestRemoved() delegate callback to
extend the lifetime of the request being removed to
OverlayPresenterImpl::OverlayWasDismissed in the instance
of a presenting WebState being detached. However, this pointer
does not guarantee the continued existence of the
|detached_presenting_request_queue_|. In addition, in this callback,
there is no need to reset the delegate since OverlayPresenterImpl
doesn't need to listen for OverlayRequestRemoved() anymore with
|presented_request_| being reset in this callback.

Bug: 1155138
Change-Id: Ie5ec9b3ee1fe85484ab51919e5f9656e3351f41b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2575318Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835313}

ios/chrome/browser/overlays/overlay_presenter_impl.mm

@@ -311,11 +311,8 @@ void OverlayPresenterImpl::OverlayWasDismissed(
   // The OverlayPresenter remains as the delegate for
   // |detached_presenting_request_queue_| to ensure that |presented_request_| is
   // not deleted before the dismissal of its UI is finished.  Since the UI is
-  // now being dismissed, the delegate can be reset.
-  if (detached_presenting_request_queue_) {
-    detached_presenting_request_queue_->SetDelegate(nullptr);
-    detached_presenting_request_queue_ = nullptr;
-  }
+  // now being dismissed, this reference is not needed anymore.
+  detached_presenting_request_queue_ = nullptr;
 
   // Notify the observers that the overlay UI was hidden.
   for (auto& observer : observers_) {