libs/events: remove unneccessary unsubscription code (#8135)

The events switch code is largely vestigal and is responsible for
wiring between the consensus state machine and the consensus
reactor. While there might have been a need, historicallly to managed
these subscriptions at runtime, it's nolonger used: subscriptions are
registered during startup, and then the switch shuts down at at the
end. 

Eventually the EventSwitch should be replaced by a much smaller
implementation of an eventloop in the consensus state machine, but
cutting down on the scope of the event switch will help clarify the
requirements from the consensus side.

internal/consensus/reactor.go

@@ -219,8 +219,6 @@ func (r *Reactor) OnStart(ctx context.Context) error {
 // blocking until they all exit, as well as unsubscribing from events and stopping
 // state.
 func (r *Reactor) OnStop() {
-	r.unsubscribeFromBroadcastEvents()
-
 	r.state.Stop()
 
 	if !r.WaitSync() {
@@ -394,10 +392,6 @@ func (r *Reactor) subscribeToBroadcastEvents() {
 	}
 }
 
-func (r *Reactor) unsubscribeFromBroadcastEvents() {
-	r.state.evsw.RemoveListener(listenerIDConsensus)
-}
-
 func makeRoundStepMessage(rs *cstypes.RoundState) *tmcons.NewRoundStep {
 	return &tmcons.NewRoundStep{
 		Height:                rs.Height,