Merge #17285: doc: Bip70 removal follow-up

3ed8e3d079 doc: Remove explicit network name references (Fabian Jahr)
d6e493f0c2 wallet: Remove left-over BIP70 comment (Fabian Jahr)

Pull request description:

  A small follow-up to #17165 which removed BIP70 support.

  1. Removes one leftover mention of BIP70 in a comment.
  2. 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.

ACKs for top commit:
  MarcoFalke:
    ACK 3ed8e3d079

Tree-SHA512: 9a7c0b9cacbb67bd31a089ffdc6f1ebc7f336493e2c8266eb697da34dce2b505a431d5639a3e4fc34f9287361343e861b55dc2662e0a1d2095cc1046db77d6ee

src/util/system.h

@@ -265,7 +265,7 @@ public:
     void ForceSetArg(const std::string& strArg, const std::string& strValue);
 
     /**
-     * Looks for -regtest, -testnet and returns the appropriate BIP70 chain name.
+     * Returns the appropriate chain name from the program arguments.
      * @return CBaseChainParams::MAIN by default; raises runtime error if an invalid combination is given.
      */
     std::string GetChainName() const;