Keep `im-online` events decodable after pallet removal #1018

Job	Run time
Extract tasks from matrix	6s
check-migrations (polkadot, polkadot-runtime, relay/polkadot, wss://polkadot-try-runtime-node.par...	17m 4s
check-migrations (kusama, staging-kusama-runtime, relay/kusama, wss://kusama-try-runtime-node.par...	13m 56s
check-migrations (asset-hub-kusama, asset-hub-kusama-runtime, system-parachains/asset-hubs/asset-...	6m 42s
check-migrations (asset-hub-polkadot, asset-hub-polkadot-runtime, system-parachains/asset-hubs/as...	4m 57s
check-migrations (bridge-hub-kusama, bridge-hub-kusama-runtime, system-parachains/bridge-hubs/bri...	4m 24s
check-migrations (bridge-hub-polkadot, bridge-hub-polkadot-runtime, system-parachains/bridge-hubs...	4m 25s
check-migrations (collectives-polkadot, collectives-polkadot-runtime, system-parachains/collectiv...	5m 11s
check-migrations (encointer-kusama, encointer-kusama-runtime, system-parachains/encointer, wss://...	5m 13s
	1h 1m 58s

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