remove unused code from demobrowser

Remove QWEBENGINEPAGE_ISMODIFIED block from demobrowser. This feature is
not implemented. It is questionable whether it should be re-introduced
in its old form.
Checking whether the user should think twice about leaving the current
page is usually implemented by the HTML page itself.

Change-Id: I51544129b26f3e0c132e2c983c2ce1744cc19123
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>