beacon/light/sync: clear reqFinalityEpoch on server unregistration (#33483)

HeadSync kept reqFinalityEpoch entries for servers after receiving
EvUnregistered, while other per-server maps were cleared. This left
stale request.Server keys reachable from HeadSync, which can lead to a
slow memory leak in setups that dynamically register and unregister
servers.

The fix adds deletion of the reqFinalityEpoch entry in the
EvUnregistered handler. This aligns HeadSync with the cleanup pattern
used by other sync modules and keeps the finality request bookkeeping
strictly limited to currently registered servers.

beacon/light/sync/head_sync.go

@@ -105,6 +105,7 @@ func (s *HeadSync) Process(requester request.Requester, events []request.Event)
 			delete(s.serverHeads, event.Server)
 			delete(s.unvalidatedOptimistic, event.Server)
 			delete(s.unvalidatedFinality, event.Server)
+			delete(s.reqFinalityEpoch, event.Server)
 		}
 	}
 }