st/scroll-view: Don't emit superfluous style-changed events on children

st/scroll-view: Don't emit superfluous style-changed events on children

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>