remove leftovers of the contracts-rococo parachain (#7638)

There were already previous efforts to remove the contracts-rococo
chain, see #5471, which was done as a response to this comment
https://github.com/paritytech/polkadot-sdk/pull/5288#discussion_r1711157476.

This PR intends to fix the parts that were overlooked back then, and
remove all traces of contracts-rococo as it is intended to be replaced
by a new testnet including pallet-revive.