Keep `im-online` events decodable after pallet removal #1314

Job	Run time
Extract runtimes from matrix	4s
Extract integration tests from matrix	3s
build-chain-spec-generator	9m 36s
integration-test (asset-hub-kusama, asset-hub-kusama-integration-tests)	34m 53s
integration-test (asset-hub-polkadot, asset-hub-polkadot-integration-tests)	37m 38s
integration-test (bridge-hub-kusama, bridge-hub-kusama-integration-tests)	39m 30s
integration-test (bridge-hub-polkadot, bridge-hub-polkadot-integration-tests)	39m 26s
runtime-test (polkadot, polkadot-runtime, relay/polkadot, wss://polkadot-try-runtime-node.parity-...	15m 7s
runtime-test (kusama, staging-kusama-runtime, relay/kusama, wss://kusama-try-runtime-node.parity-...	15m 36s
runtime-test (glutton-kusama, glutton-kusama-runtime, system-parachains/gluttons/glutton-kusama, ...	8m 36s
runtime-test (asset-hub-kusama, asset-hub-kusama-runtime, system-parachains/asset-hubs/asset-hub-...	8m 2s
runtime-test (asset-hub-polkadot, asset-hub-polkadot-runtime, system-parachains/asset-hubs/asset-...	7m 48s
runtime-test (bridge-hub-kusama, bridge-hub-kusama-runtime, system-parachains/bridge-hubs/bridge-...	5m 36s
runtime-test (bridge-hub-polkadot, bridge-hub-polkadot-runtime, system-parachains/bridge-hubs/bri...	5m 44s
runtime-test (collectives-polkadot, collectives-polkadot-runtime, system-parachains/collectives/c...	6m 24s
runtime-test (encointer-kusama, encointer-kusama-runtime, system-parachains/encointer, wss://kusa...	6m 40s
	4h 0m 43s

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