Avoid double overwrite check

Summary:
QFileDialog has by default an overwrite check, avoid the second one done by kate.

Related to D21401

Reviewers: #kate, #frameworks, cullmann

Reviewed By: #kate, cullmann

Subscribers: apol, dhaumann, kwrite-devel

Tags: #kate

Differential Revision: https://phabricator.kde.org/D21402