polkadot-parachain: one chain-spec for all (#2457)

This PR removes some `ChainSpec` types which are not necessary
(left-overs from #1256). Currently `ChainSpec` does not have to be
generic over the specific `RuntimeGenesisConfig`, it is enough to use
single type for all:

https://github.com/paritytech/polkadot-sdk/blob/9f787018/cumulus/polkadot-parachain/src/chain_spec/mod.rs#L53-L54


related to: https://github.com/paritytech/polkadot-sdk/issues/25

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Co-authored-by: command-bot <>