knot: use upstream RemoveSource to drop revoked knot subscriptions
Bump tangled.org/core to a version that exposes
eventconsumer.Consumer.RemoveSource, and replace the placeholder
no-op body of knotConsumer.RemoveKnot with a real call.
Previously, when a repo revoked our spindle membership we logged the intent
but left the underlying websocket dialing the now-untrusted knot until
tack restarted; reconciliation could only ever add sources, never
remove them. RemoveKnot now tears the source down immediately, and the
tracking note in KNOWN_ISSUES.md plus the workaround comments referencing the
upstream PR are no longer accurate, so they are removed.
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