Remove origin from unsigned_pdu_base.yaml (#998)

It doesn't serve a useful purpose, is not enforced to be present by Synapse
and already being omitted by at least one homeserver implementation.

changelogs/server_server/newsfragments/998.clarification

@@ -0,0 +1 @@
+Remove `origin` field from PDUs which exists on many but not all PDUs in practice and doesn't serve an actual purpose.

data/api/server-server/definitions/unsigned_pdu_base.yaml

@@ -25,10 +25,6 @@ properties:
     type: string
     description: The ID of the user sending the event.
     example: "@someone:matrix.org"
-  origin:
-    type: string
-    description: The `server_name` of the homeserver that created this event.
-    example: "matrix.org"
   origin_server_ts:
     type: integer
     format: int64
@@ -128,7 +124,6 @@ required:
   - event_id
   - room_id
   - sender
-  - origin
   - origin_server_ts
   - type
   - content