Doc: Removing url variable from qdocconf file.

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

Change-Id: I94a0da7aa98af1fdd6140168fe9feb27b11c0bdb
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>