They're either wrong (when using FALSE because the widgets don't account
for changes to the CSS) or unnecessary (when using TRUE because it's the
default).
Fixes!1777
It tried to set the expand state if either xexpand/yexpand where true.
Due to a missing queue_compute_expand when adding a child it actually
only computed the expand state in case a child queued after being added
or in case a child had the expand property set (see optimization in
gtk_widget_set_parent)
In my case this broke layouts as a child of GtkCombBox started setting
an exand flag with 3.20 which queued a compute_expand, which in turn
propagated an expand child props set for a cell in the same table up
and overrode the expand child prop of a parent GtkBox.
This removes the custom compute_expand implementation to match the
behaviour of GtkBox (don't propagate child prop expand flags
but let child expand flags override the child props) and not get random
expand behaviour depending on whether and when child widgets set their
expand state.
https://bugzilla.gnome.org/show_bug.cgi?id=769162
