[Treasurer] Fix and Add Asset Rates

This referendum does three things related to asset rates, the values used when applying track privilege to treasury spends in non-DOT assets.

1. Adds an asset rate for DED, after 50 billion DED were sent to the Treasury account. The rate set is 1 DOT = 30,000 DED.
2. Corrects the asset rates for USDT and USDC. The previous values were set in 645 using the wrong encoding of the value. TLDR: My bad.
3. Removes the asset rates for DOT. This was originally required when introducing multi-asset spends, since the logic needed a rate for everything (including DOT), but this has been fixed and spends in DOT just work.