MM-21473: Remove unused cluster events (#17914)

```release-note
Removed several cluster events:
- ClusterEventInvalidateCacheForWebhook
- ClusterEventInvalidateCacheForChannelPosts
- ClusterEventInvalidateCacheForChannelMembers
```

model/cluster_message.go

@@ -13,10 +13,7 @@ const (
 	ClusterEventUpdateStatus                                = "update_status"
 	ClusterEventInvalidateAllCaches                         = "inv_all_caches"
 	ClusterEventInvalidateCacheForReactions                 = "inv_reactions"
-	ClusterEventInvalidateCacheForWebhook                   = "inv_webhook"
-	ClusterEventInvalidateCacheForChannelPosts              = "inv_channel_posts"
 	ClusterEventInvalidateCacheForChannelMembersNotifyProps = "inv_channel_members_notify_props"
-	ClusterEventInvalidateCacheForChannelMembers            = "inv_channel_members"
 	ClusterEventInvalidateCacheForChannelByName             = "inv_channel_name"
 	ClusterEventInvalidateCacheForChannel                   = "inv_channel"
 	ClusterEventInvalidateCacheForChannelGuestCount         = "inv_channel_guest_count"