Do not remove participant from cache on disconnect. (#4241)

Resuming participants need to know about participants disconnected
during resume.

Reverts https://github.com/livekit/livekit/pull/4234

pkg/rtc/participant_signal.go

@@ -118,10 +118,6 @@ func (p *ParticipantImpl) SendParticipantUpdate(participantsToUpdate []*livekit.
 			})
 			validUpdates = append(validUpdates, pi)
 		}
-
-		if pi.State == livekit.ParticipantInfo_DISCONNECTED {
-			p.updateCache.Remove(pID)
-		}
 	}
 	p.updateLock.Unlock()