cleanup: Remove custom BoxLayout meta
Get in on the fun of cleaning out deprecated code, then change
ClutterBoxLayout to only consider the generic align/expand properties for laying out children.
Quoting the last commit:
ClutterBoxLayout's layout policy of using the generic ClutterActor align/expand properties for children that are expanded and a custom meta otherwise is confusing, in particular as the x-fill/y-fill defaults don't match the default CLUTTER_ACTOR_ALIGN_FILL align. StBoxLayout's own custom child meta (which was deprecated last cycle) is probably the only consumer. And luckily, the St meta uses different x-fill/y-fill default that match the ClutterActor defaults, so removing it will not affect code that doesn't use the deprecated properties themselves.