bip151: remove aad change.

Author: chjj

bip-0151.mediawiki

@@ -123,8 +123,6 @@ After a successful <code>encinit</code>/<code>encack</code> interaction from bot
 
 Encrypted messages do not have the 4byte network magic.
 
-The ciphertext payload length must be included in the AEAD MAC as AAD.
-
 The maximum message length needs to be chosen carefully. The 4 byte length field can lead to a required message buffer of 4 GiB.
 Processing the message before the authentication succeeds must not be done.