Remove fenced frame config mapping when urn iframes are removed

[gtanzer]

Remove fenced frame config mapping when urn iframes are gone (and the construct is no longer necessary)

[domfarolino]

One useful part of the centralized config "mapping" even after URN iframes (although the "mapping" part is less useful I suppose) is the ease of having a single place to touch when we update any "pending" configs. We'd have to find a not hand-wavey way to do this if we get rid of the centralized mapping.