Remove unrelated elements when changing item type

[luku]

This partially solves situation, where changing item type of an item keeps already filled information in elements not associated with new item type.
The problem is mostly apparent when searching, because all existing element texts are indexed, but not visible in show pages.

There's still issue when removing element association for specific item type and when removing element completely from Omeka. But it's less common case and users can be prompted to reindex search.