remove useless "block" tag on OTS.

03.md

@@ -13,15 +13,13 @@ This NIP defines an event with `kind:1040` that can contain an [OpenTimestamps](
   "kind": 1040
   "tags": [
     ["e", <event-id>, <relay-url>],
-    ["block", <block-height-as-string>, <block-hash>],
     ["alt", "opentimestamps attestation"]
   ],
   "content": <base64-encoded OTS file data>
 }
 ```
 
 - The OpenTimestamps proof MUST prove the referenced `e` event id as its digest.
-- The `block` tag is optional, it exists to establish a minimum date since which the given event has existed.
 - The `content` MUST be the full content of an `.ots` file containing at least one Bitcoin attestation. This file SHOULD contain a **single** Bitcoin attestation and no reference to "pending" attestations since they are useless in this context.
 
 ### Example OpenTimestamps proof verification flow