fix: no events with case as subject

the sub claim refers to the subject of the SET. previously
we had events whose subject is the case (case-close-event).

since we no longer have this event, the case subject is no longer
needed.

spec/set-payload.schema.json

@@ -27,7 +27,7 @@
     "sub": {
       "description": "Beschreibt, wen das SET betrifft.",
       "type": "string",
-      "pattern": "^(case|submission|reply):[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$"
+      "pattern": "^(submission|reply):[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$"
     },
     "txn": {
       "description": "Der Transaction Identifier ('txn') enthält 'case:' und die UUID des Cases.",