Remove "Mobile Data" section from quick settings when not available.

When a cellular dongle is removed and quick settings is already opened
the now empty "Mobile Data" section was previously shifting to the
bottom instead of being removed.

Bug: 812337
Change-Id: I5082e383220e7c6bb66a34e2435425cbf4dbdd20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1670412Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Jon Mann <jonmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671378}

ash/system/network/network_list_view.cc

@@ -259,6 +259,11 @@ NetworkListView::UpdateNetworkListEntries() {
       ++index;
     index += new_cellular_guids->size();
     new_guids->insert(new_cellular_guids->begin(), new_cellular_guids->end());
+  } else if (mobile_header_view_) {
+    scroll_content()->RemoveChildView(mobile_header_view_);
+    delete mobile_header_view_;
+    mobile_header_view_ = nullptr;
+    needs_relayout_ = true;
   }
 
   if (!wifi_header_view_)