A small follow-up to #17165 which removed BIP70 support.
- Removes one leftover mention of BIP70 in a comment.
- Removes BIP70 reference in comments on network/chain name strings. These can be removed as they are not really helpful and also incorrect: BIP70 only defines "main" and "test" but not "regtest". If/When signet gets merged we will add another name to the list that is not defined in BIP70. Mostly there is also an exhaustive list of the options included in the comment anyway.
If we would like to keep an identifier for this naming scheme, I would suggest switching to something more generic, like 'short chain name'. Happy to implement that if that is preferred. Alternatively, we could add a reference to CBaseChainParams. That would also mean we don't have to change these lines again for signet.