Add XCM pallet and config to Polkadot runtime (#4313)

* xcm added

* xcm pallet & config added Polkadot

* set advertise XCM version to 2 for Polkadot

* remove ChildSystemParachainAsSuperuser from Polkadot XCM config

* remove unpaid execution for system parachains

* remove unused imports

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

polkadot/node/service/src/chain_spec.rs

@@ -366,6 +366,7 @@ fn polkadot_staging_testnet_config_genesis(wasm_binary: &[u8]) -> polkadot::Gene
 			config: default_parachains_host_configuration(),
 		},
 		paras: Default::default(),
+		xcm_pallet: polkadot::XcmPalletConfig { safe_xcm_version: Some(2) },
 	}
 }
 
@@ -1310,6 +1311,7 @@ pub fn polkadot_testnet_genesis(
 			config: default_parachains_host_configuration(),
 		},
 		paras: Default::default(),
+		xcm_pallet: polkadot::XcmPalletConfig { safe_xcm_version: Some(2) },
 	}
 }