Removing a branch at Parallel does not clean up underlying resources

[odacremolbap]

Describe the bug
Removing a branch in a parallel object does not remove underlying resources

Expected behavior
Removing a branch in a parallel object should remove the no longer used channel and subscription.
This same issue was recently fixed for sequences.

To Reproduce
Follow docs to create a Parallel object with at least 2 branches:
https://knative.dev/docs/eventing/samples/parallel/multiple-branches/

Expected subscriptions and channels look like this:

$ kubectl get subscriptions.messaging.knative.dev 
NAME                                     AGE    READY   REASON
odd-even-parallel-kn-parallel-0          8m9s   True    
odd-even-parallel-kn-parallel-1          8m9s   True    
odd-even-parallel-kn-parallel-filter-0   8m9s   True    
odd-even-parallel-kn-parallel-filter-1   8m9s   True 

$ kubectl get inmemorychannels.messaging.knative.dev 
NAME                              URL                                                                           AGE     READY   REASON
odd-even-parallel-kn-parallel     http://odd-even-parallel-kn-parallel-kn-channel.default.svc.cluster.local     7m56s   True    
odd-even-parallel-kn-parallel-0   http://odd-even-parallel-kn-parallel-0-kn-channel.default.svc.cluster.local   7m56s   True    
odd-even-parallel-kn-parallel-1   http://odd-even-parallel-kn-parallel-1-kn-channel.default.svc.cluster.local   7m56s   True    

Remove one branch from the parallel and list channels and subscriptions again. The first branch's subscription has been re-defined (see AGE column) but the removed branch subscription and channel are still present but unused:

$ kubectl get subscriptions.messaging.knative.dev 
NAME                                     AGE     READY   REASON
odd-even-parallel-kn-parallel-0          23s     True    
odd-even-parallel-kn-parallel-1          9m33s   True    
odd-even-parallel-kn-parallel-filter-0   23s     True    
odd-even-parallel-kn-parallel-filter-1   9m33s   True    

kubectl get inmemorychannels.messaging.knative.dev 
NAME                              URL                                                                           AGE     READY   REASON
odd-even-parallel-kn-parallel     http://odd-even-parallel-kn-parallel-kn-channel.default.svc.cluster.local     9m22s   True    
odd-even-parallel-kn-parallel-0   http://odd-even-parallel-kn-parallel-0-kn-channel.default.svc.cluster.local   9m22s   True    
odd-even-parallel-kn-parallel-1   http://odd-even-parallel-kn-parallel-1-kn-channel.default.svc.cluster.local   9m22s   True    

Knative release version
Eventing v0.24

[odacremolbap]

/assign