Remove fallback price from asset registry

As we are now using router and oracles for transaction fee payments and have insufficient assets, we no longer need fallback price since it would mean we could pay ED or any fees with outdated prices. It is better to not do so.

In the future we can improve this so that any ( HDX <> Asset ) XYK pool could serve as payment source with direct swap during a transaction similar to AH asset conversion pallet.