Revert "@:keep Scene's constructor to workaround reflection issues."

This reverts commit 559f1f3174d3f95f8304ad2f11c62416f5745968.

Fixed upstream at HaxeFoundation/haxe#2121. The next Haxe release is
probably going to be before the next Flambe release, so revert this here
workaround. Fixes #162.