Respect chainId from TransactionArgs when signing

[yglukhov]

This fixes the problem of assertion failure in proc send*(web3: Web3, c: TransactionArgs, chainId: ChainId), and eliminates the need for this function along with func encodeTransaction*(s: TransactionArgs, pk: PrivateKey, chainId: ChainId): seq[byte] as they are redundant because chainId option already exists in TransactionArgs but was not respected during the signature. Thus these functions are marked deprecated now.

The aforementioned assertion failure happens when user provides the chainId without the privateKey, which IMO should not affect each other in the high level.