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/9f787018857660440182142adc806954d7d07709/cumulus/polkadot-parachain/src/chain_spec/mod.rs#L53-L54


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

---------

Co-authored-by: command-bot <>