remove deprecated type 'GenesisConfig' (#3378)

# Description

Removed deprecated type `GenesisConfig` from the codebase.

Closes https://github.com/paritytech/polkadot-sdk/issues/175

# Checklist

- [x] My PR includes a detailed description as outlined in the
"Description" section above
- [x] My PR follows the [labeling requirements](CONTRIBUTING.md#Process)
of this project (at minimum one label for `T`
  required)
- [x] I have made corresponding changes to the documentation (if
applicable)

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Co-authored-by: Liam Aharon <[email protected]>
Co-authored-by: Michal Kucharczyk <[email protected]>