Since we correctly call the `style_changed` vfunc superclass at the end
of our own function anyway, the style changes will get propagated to the
children of the scrollView inside `st_widget_real_style_changed` anyway.
So remove those unneeded and quite expensive (because they cause the
theme node to be regenerated) calls to `st_widget_style_changed`.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/953>
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