Removed flash-specific untyped property set

The line "untyped details.target[details.propertyName] = value" in
SimpleActuator.hx effectively bypasses property setters/getters, whereas
setField/setProperty do not, at least in Haxe3. Pretty weird behavior on the haxe end, I think, but still.

This causes an issue where tweening properties on the Flash target do not call setter/getter functions.
Removing it fixes this issue.