Remove UnknownStatement.getExtensionDefinition()

UnknownSchemaNode has an unfortunate design flaw, where it can recursively refer to itself if there are other UnknownSchemaNode in its ExtensionDefinition (and those use that statement).

YANGTOOLS-1196 effectively eliminated this use case from our codebase, as we do not support any extensions which would have such interdependencies, but there are such extensions out there and those cannot be reasonably implemented now.

The crux of the problem is UnknownSchemaNode.getExtensionDefinition(), which is not really useful, as we already have the corresponding QName and the extension can easily be looked up from EffectiveModelContext. Remove this method.