kdeplatformfiledialoghelper: don't create our own QDialogButtonBox
With frameworks/kio!1243 (closed), kfilewidget will have its own QDialogButtonBox. So all we'll need to do it unhide its buttons and connect them to the appropriate signal handlers. No need to manufacture our own QDialogButtonBox anymore.
Technically this is backportable to Plasma 5.27, since nothing gets broken if a user has this change but not frameworks/kio!1243 (closed). However, in that circumstance, the file dialog shows its accept and reject buttons vertically beside the filename and filter fields, which looks a bit odd. So such a user would experience their file dialogs briefly looking like that, and then when they upgrade to the KF5 version that ultimately includes frameworks/kio!1243 (closed), it will return back to how it looked before. So it may be wise to leave it as a Plasma-6-only change.