Add validate_xcm_nesting to the ParentAsUmp and ChildParachainRouter (#4236)

This PR:
- moves `validate_xcm_nesting` from `XcmpQueue` into the `VersionedXcm`
- adds `validate_xcm_nesting` to the `ParentAsUmp`
- adds `validate_xcm_nesting` to the `ChildParachainRouter`


Based on discussion
[here](https://github.com/paritytech/polkadot-sdk/pull/4186#discussion_r1571344270)
and/or
[here](https://github.com/paritytech/polkadot-sdk/pull/4186#discussion_r1572076666)
and/or [here]()

## Question/TODO

- [x] To the
[comment](https://github.com/paritytech/polkadot-sdk/pull/4186#discussion_r1572072295)
- Why was `validate_xcm_nesting` added just to the `XcmpQueue` router
and nowhere else? What kind of problem `MAX_XCM_DECODE_DEPTH` is
solving? (see
[comment](https://github.com/paritytech/polkadot-sdk/pull/4236#discussion_r1574605191))

cumulus/pallets/xcmp-queue/src/lib.rs

@@ -916,7 +916,8 @@ impl<T: Config> SendXcm for Pallet<T> {
 				let price = T::PriceForSiblingDelivery::price_for_delivery(id, &xcm);
 				let versioned_xcm = T::VersionWrapper::wrap_version(&d, xcm)
 					.map_err(|()| SendError::DestinationUnsupported)?;
-				validate_xcm_nesting(&versioned_xcm)
+				versioned_xcm
+					.validate_xcm_nesting()
 					.map_err(|()| SendError::ExceedsMaxMessageSize)?;
 
 				Ok(((id, versioned_xcm), price))
@@ -932,10 +933,6 @@ impl<T: Config> SendXcm for Pallet<T> {
 
 	fn deliver((id, xcm): (ParaId, VersionedXcm<()>)) -> Result<XcmHash, SendError> {
 		let hash = xcm.using_encoded(sp_io::hashing::blake2_256);
-		defensive_assert!(
-			validate_xcm_nesting(&xcm).is_ok(),
-			"Tickets are valid prior to delivery by trait XCM; qed"
-		);
 
 		match Self::send_fragment(id, XcmpMessageFormat::ConcatenatedVersionedXcm, xcm) {
 			Ok(_) => {
@@ -950,16 +947,6 @@ impl<T: Config> SendXcm for Pallet<T> {
 	}
 }
 
-/// Checks that the XCM is decodable with `MAX_XCM_DECODE_DEPTH`.
-///
-/// Note that this uses the limit of the sender - not the receiver. It it best effort.
-pub(crate) fn validate_xcm_nesting(xcm: &VersionedXcm<()>) -> Result<(), ()> {
-	xcm.using_encoded(|mut enc| {
-		VersionedXcm::<()>::decode_all_with_depth_limit(MAX_XCM_DECODE_DEPTH, &mut enc).map(|_| ())
-	})
-	.map_err(|_| ())
-}
-
 impl<T: Config> FeeTracker for Pallet<T> {
 	type Id = ParaId;