Remove duplicate overwrite confirmation dialog

Summary:
When trying to overwrite a file using e.g. {nav File > Save As}
the confirmation dialog is duplicated.
This patch removes the additional Gwenview overwrite check since
`QFileDialog` shows its own dialog to confirm overwrite.

FIXED-IN: 18.04.1

Test Plan:
1. Start Gwenview in View or Browse Mode
2. {nav File > Save As} or {key Ctrl Shift S}
3. Select an existing file as destination
4. After confirmation no other dialog should ask again

Reviewers: #gwenview, ngraham, rkflx

Reviewed By: #gwenview, ngraham, rkflx

Subscribers: rkflx, huoni, ngraham

Differential Revision: https://phabricator.kde.org/D12346