brontide: the encrypted packet length is no longer the associated data
brontide: the encrypted packet length is no longer the associated data

This commit modifies the current implementation to more closely match
what’s currently specified within the spec.

The encrypted+MAC’d packet length is no longer included as the
associated data for the encryption/decryption of transport messages.
This isn’t required as if an active attacker swaps out the encrypted
length in the byte string, the decryption+MAC check will simply fail as
the nonce won’t be in proper sequence.

ddustin/lnd,AndrewSamokhvalov/lnd,LightningNetwork/lnd,bryanvu/lnd,ddustin/lnd,aakselrod/lnd,aakselrod/lnd,bryanvu/lnd,AndrewSamokhvalov/lnd,LightningNetwork/lnd