bolt2: forget the check about the update_* messages.

The rational around this commit is that a node should not care about what he receive but only about what it must not receive.

The actual Close Channel spec tell us exactly when we need to fail, we need just to remove the confusing part related to the `update_*`

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

Author: vincenzopalazzo

02-peer-protocol.md

@@ -393,7 +393,7 @@ The sender:
 The recipient:
   - if `signature` is incorrect OR non-compliant with LOW-S-standard rule<sup>[LOWS](https://github.com/bitcoin/bitcoin/pull/6769)</sup>:
     - MUST send a `warning` and close the connection, or send an
-      `error` and fail the channel.
+     `error` and fail the channel.
 
 #### Rationale
 
@@ -516,7 +516,8 @@ Closing happens in two stages:
         |       |<-(2)-----  shutdown  --------|       |
         |       |                              |       |
         |       | <complete all pending HTLCs> |       |
-        |   A   |                 ...          |   B   |
+        |       |    <must not add HTLCs>      |       |
+        |   A   |              ...             |   B   |
         |       |                              |       |
         |       |--(3)-- closing_signed  F1--->|       |
         |       |<-(4)-- closing_signed  F2----|       |
@@ -545,13 +546,10 @@ A sending node:
   - if there are updates pending on the receiving node's commitment transaction:
     - MUST NOT send a `shutdown`.
   - MUST NOT send an `update_add_htlc` after a `shutdown`.
-  - if no HTLCs remain in either commitment transaction:
-    - MUST NOT send any `update` message after a `shutdown`.
   - SHOULD fail to route any HTLC added after it has sent `shutdown`.
   - if it sent a non-zero-length `shutdown_scriptpubkey` in `open_channel` or `accept_channel`:
     - MUST send the same value in `scriptpubkey`.
   - MUST set `scriptpubkey` in one of the following forms:
-
     1. `OP_0` `20` 20-bytes (version 0 pay to witness pubkey hash), OR
     2. `OP_0` `32` 32-bytes (version 0 pay to witness script hash), OR
     3. if (and only if) `option_shutdown_anysegwit` is negotiated: