Improves XCM barrier for DescendOrigin

[Agusrodri]

This PR removes our custom barrier AllowTopLevelPaidExecutionDescendOriginFirst and uses the polkadot barrier WithComputedOrigin instead.

Our custom barrier AllowTopLevelPaidExecutionDescendOriginFirst was originally created to test xcm->evm on moonbase as a copy of AllowTopLevelPaidExecutionFrom but with DescendOrigin instead of ClearOrigin.

At the time, polkadot didn't offer any barriers to support DescendOrigin instruction, so it made sense to create our own, but that's no longer the case.

Parity has developed a generic barrier WithComputedOrigin that wraps existing barriers and handle the prefixed DescendOrigin instruction. Now, all relays and system parachains like statemine use this new barrier this way:

pub type XcmBarrier = (
	// Weight that is paid for may be consumed.
	TakeWeightCredit,
	// Expected responses are OK.
	AllowKnownQueryResponses<XcmPallet>,
	WithComputedOrigin<
		(
			// If the message is one that immediately attemps to pay for execution, then allow it.
			AllowTopLevelPaidExecutionFrom<Everything>,
                        ...
			// Subscriptions for version tracking are OK.
			AllowSubscriptionsFrom<Filter>,
		),
		UniversalLocation,
		ConstU32<8>, // max prefixed instructions (DescendOrigin or UniversalOrigin)
	>,
);

[librelois]

@Agusrodri @girazoki I think we should remove our custom AllowTopLevelPaidExecutionDescendOriginFirst barrier, and use WithComputedOrigin like polkadot: https://github.com/paritytech/polkadot/blame/master/runtime/polkadot/src/xcm_config.rs#L163

[github-actions]

Coverage generated "Thu Jun 8 08:40:14 UTC 2023":
https://s3.amazonaws.com/moonbeam-coverage/pulls/2335/html/index.html

Master coverage: 71.22%
Pull coverage: 71.21%

[girazoki]

Fully agree @librelois, I forgot about these new Barrier