demobrowser: Remove unused loadingUrl

Remove the unused m_loadingUrl member.
Remove the loadingUrl signal that was forwarded to urlChanged.
QWebEngineView already emits urlChanged. There's no need to do this twice.

Change-Id: Ib9734c5b1571745f7b82b4e5f2bc92c7f1d8f51c
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>