KH-190: Add unique filter for codes

Helps to prevent duplicates from showing. Though better way is probably
not to return duplicates at all through json...
OVT-7280: Delete relation between codes

Had to remove KoodistonSuhde from KoodistoVersio's ylakoodistot and
alakoodistot sets first before removing. Which is more than a little
perplexing.
Fix test