Replace DEX pallet by Asset Conversion pallet

[stiiifff]

Trappist includes a DEX pallet implementation of the Uniswap V1 protocol. This pallet was built by a DS engineer as an onboarding project. It has not been battle-tested in a production environment, and has not passed security audits. Additionally, it creates some maintenance effort every time the Trappist runtime is updated to a new Polkadot release.

On the other hand, the Substrate FRAME library now includes the Asset Conversion pallet which implements the Uniswap V2 protocol, is being used as the main DEX on the Asset Hub, and has gone through extensive peer reviews and a security audit(s).

We should therefore replace Trappist’s DEX implementation by the new Asset Conversation pallet, and archive the old DEX pallet’s repository.