optimizer: Remove unused flag added in d14b4a8
racket/gui: change placement of children in `panel%` and `pane%`

The `panel%` and `pane%` classes were not originally intended for
direct instantiation; instead, `horizontal-pane[l]%` and
`vertical-pane[l]%` provide basic placements that make sense for
multiple children. Adjusting `pane[l]%` to just overlay all children
seems both sensible and useful (if only one child is shown at a time,
for example).

As suggested by David Nelson.