bolt04: Remove TLV based termination signal

As discussed during the IRC meeting on 2019-07-22 this would have been a
duplication of signals. It was decided to use one for now, with the option of
coming back should we ever need the last 32 bytes of the onion.

04-onion-routing.md

@@ -245,7 +245,6 @@ This is a more flexible format, which avoids the redundant `short_channel_id` fi
 
 1. tlvs: `tlv_payload`
 2. types:
-    1. type: 0 (`destination_signal`)
     1. type: 2 (`amt_to_forward`)
     2. data:
         * [`tu64`:`amt_to_forward`]