Refactored delete index api to make use of the new recently introduced generic ack mechanism

Refactored delete index api to make use of the new recently introduced generic ack mechanism

Side note: the double notification from each node (one after deleting from metadata, one after deleting from file system) that was in place is not needed anymore, as each node returns the ack in the clusterStateProcessed, which gets called after all the listeners (one of which actually deletes from file system)

Closes #4217