assistant: remove a TODO that's done

TBR=xiaohuic@chromium.org

Change-Id: I61ca7fc31d82645e0b49d0083438a6a53531ed39
Reviewed-on: https://chromium-review.googlesource.com/1145731Reviewed-by: Muyuan Li <muyuanli@chromium.org>
Commit-Queue: Muyuan Li <muyuanli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576988}

chromeos/services/assistant/assistant_manager_service_impl.cc

@@ -190,7 +190,6 @@ void AssistantManagerServiceImpl::RequestScreenContext(
       base::BindOnce(&AssistantManagerServiceImpl::OnAssistantStructureReceived,
                      weak_factory_.GetWeakPtr(), on_done));
 
-  // TODO(muyuanli): handle metalayer and grab only part of the screen.
   service_->assistant_controller()->RequestScreenshot(
       region, base::BindOnce(
                   &AssistantManagerServiceImpl::OnAssistantScreenshotReceived,