Asset Hubs: switch over to paying fees using auto-swaps

[acatangiu]

TakeFirstAssetTrader is still being used on Asset Hubs. This should be removed and rely solely on SwapFirstAssetTrader for paying fees with other assets than DOT/KSM.

TakeFirstAssetTrader was meant as a temporary solution until we have SwapFirstAssetTrader working and live swap pools with popular assets. The conditions are now right to remove TakeFirstAssetTrader.

The motivation to remove TakeFirstAssetTrader is that it doesn't (and can't) correctly account the economic value of weight. Depending on market movements, buying weight with sufficient assets can be abused to potentially severely undercut fees.