Remove handling of privilege operation confirmation prompts from KIO::Job

Summary:
Letting KIO::Job handle these prompts is pointless because an application can always choose not to show these prompts
(by connecting privilegeOperationRequested signal to an empty slot) thus compromising security.

Test Plan: All the tests in privilegejobtest pass.

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: fvogt, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D10567