Remove caching for property datatype dropdowns

The caching here seems to cause some confusion (#241, #207).  If you've added a new datatype, it won't show in Archetype's datatype dropdown until a hard-refresh, yet it does in Umbraco's.

Since this method is only used in the "backend" (ie, the Developer/Datatype section and not the Archetype UI), I think it's probably OK not to cache it.