Appwizard: fix broken disconnect in ProjectVcsPage

Summary:
The disconnect was broken since it was added:
    disconnect( this, SLOT(validateData()) );
(added in 9f46f693)
which will call
    QObject::disconnect(const QObject *receiver, const char *method)
    "Disconnects all signals in this object from receiver's method."
and thus not do what has been intended, to disconnect from the signal
of the previous importWidget.
The port to new signal/slot syntax did not improve the wrong logic
even if the very disconnect call was discussed for other reasons
in the review :)
(see commit f6267438 and
https://git.reviewboard.kde.org/r/121045)

The broken disconnect did not result in a real bug from what I saw,
but still added noise in the console log. No longer.

Reviewers: #kdevelop, kfunk

Reviewed By: #kdevelop, kfunk

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D5536