`im-online` removal final cleanup (#3902)

Rejoice! Rejoice! The story is nearly over.

This PR removes stale migrations, auxiliary structures, and package
dependencies, thus making Rococo and Westend totally free from any
`im-online`-related stuff.

`im-online` still stays a part of the Substrate node and its runtime:
https://github.com/paritytech/polkadot-sdk/blob/0d932484/substrate/bin/node/runtime/src/lib.rs#L2276-L2277
I'm not sure if it makes sense to remove it from there considering that
we're not removing `im-online` from FRAME. Please share your opinion.