Remove `Aes256Gcm` from `GenericCipher`

[jbesraa]

It is already marked as dead_code https://github.com/stratum-mining/stratum/blob/main/protocols/v2/noise-sv2/src/cipher_state.rs#L181 and I think we are only using the CC20Poly cipher

[jbesraa]

Removing Aes256Gcm would leave GenericCipher with only the CC20P property. This raises the question of whether GenericCipher is still necessary, or if we should just use CC20P directly instead.

[Shourya742]

Adding Plebhash's comment during noise documentation here:(#1130 (comment)).