Unfollowing a follower index that has had its remote cluster deleted throws 500 error

[cjcenizal]

Discovered via elastic/kibana#68711. To reproduce, create a follower index, remove its remote cluster, and then try to unfollow the follower index.

# Delete remote
PUT _cluster/settings
{
  "persistent": {
    "cluster": {
      "remote": {
        "test3": { 
          "mode": null,
          "seeds": null,
          "skip_unavailable": null,
          "transport": {
            "compress": null
          }
        }
      }
    }
  }
}

# Pause, close, and unfollow the follower index
POST /my-index/_ccr/pause_follow
POST /my-index/_close
# This will timeout
POST /my-index/_ccr/unfollow

The ES logs will show this type of error and stacktrace:

   │ info [o.e.c.s.MasterService] [CJs-MacBook-Pro] exception thrown by listener while notifying of cluster state processed from [unfollow_action], old cluster state:
   │      cluster uuid: 7r3ijP6HT-2QY81Ne2x4kQ [committed: true]
   │      version: 82
   │      state uuid: 5BkNXu6gQD6GZaqRHwv26Q
   │      from_diff: false
   │      meta data version: 79

<snip>

   │      customs:
   │         security_tokens: TokenMetadata{ everything is secret }   restore: RestoreInProgress]
   │      org.elasticsearch.transport.NoSuchRemoteClusterException: no such remote cluster: [test2]
   │      	at org.elasticsearch.transport.RemoteClusterService.getRemoteClusterClient(RemoteClusterService.java:388) ~[elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │      	at org.elasticsearch.client.node.NodeClient.getRemoteClusterClient(NodeClient.java:156) ~[elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │      	at org.elasticsearch.xpack.ccr.action.TransportUnfollowAction$1.clusterStateProcessed(TransportUnfollowAction.java:101) ~[?:?]
   │      	at org.elasticsearch.cluster.service.MasterService$SafeClusterStateTaskListener.clusterStateProcessed(MasterService.java:518) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │      	at org.elasticsearch.cluster.service.MasterService$TaskOutputs.lambda$processedDifferentClusterState$1(MasterService.java:405) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │      	at java.util.ArrayList.forEach(ArrayList.java:1511) [?:?]
   │      	at org.elasticsearch.cluster.service.MasterService$TaskOutputs.processedDifferentClusterState(MasterService.java:405) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │      	at org.elasticsearch.cluster.service.MasterService.onPublicationSuccess(MasterService.java:265) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │      	at org.elasticsearch.cluster.service.MasterService.publish(MasterService.java:257) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │      	at org.elasticsearch.cluster.service.MasterService.runTasks(MasterService.java:234) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │      	at org.elasticsearch.cluster.service.MasterService$Batcher.run(MasterService.java:140) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │      	at org.elasticsearch.cluster.service.TaskBatcher.runIfNotProcessed(TaskBatcher.java:139) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │      	at org.elasticsearch.cluster.service.TaskBatcher$BatchedTask.run(TaskBatcher.java:177) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │      	at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:669) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │      	at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.runAndClean(PrioritizedEsThreadPoolExecutor.java:241) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │      	at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.run(PrioritizedEsThreadPoolExecutor.java:204) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │      	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130) [?:?]
   │      	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630) [?:?]
   │      	at java.lang.Thread.run(Thread.java:831) [?:?]

[elasticmachine]

Pinging @elastic/es-distributed (Team:Distributed)