Doc: Removing url variable from qdocconf file.

-url inherited from the url variable set in qtbase/doc/global

Change-Id: I6a09e5862558629338da4bc8332e532d77de7d49
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>