[kcmkwin/kwindesktop] Drop local back for Formlayout label width

This hack was added in the past to prevent the combobox from being
pushed out of view when the label next to it was very long, as it can be
in certain languages. However it had the effect of making the label get
elided in *all* languages, and also the issue it was working around is
no longer present; even with very long text, the combobox no longer gets
pushed out of view, and instead the formlayout first switches to mobile
view and then elides the Label, exactly as intended. So we can safely
remove the hack.

BUG: 438504
FIXED-IN: 5.22.2
(cherry picked from commit 77affd32)