KFileWidget: KF6, fix view size glitch on initial show

When the open file dialog is first shown, the icon/details...etc view is very small in size, taking a corner of the KDirOperator view; switching to another view icons to details or vice-versa fixes the issue and let the item view take the whole space as expected.

Tracked it down to this setSizePolicy() call, since removing that call doesn't seem to break anything (in KF5 and KF6), then no need to miss with the widgets size policy.