Fix ChatGPT: refactor contract to work in the context of a confidential request

[haythemsellami]

If my understanding is correct, the current implementation is not working on Rigil testnet when sending a confidential request that uses an instance of ChatGPT, more on this issue here https://github.com/flashbots/suave-geth/issues/243.

This PR refactor ChatGPT.sol to not store the apiKey in storage and just assume it will be stored confidentially somewhere else and passed as an argument to the complete() function.