Deletes an existing index. An index can only be deleted if it is in a READY or ERROR state. On successful execution of the request, the index will be in a DELETING state. And on completion of the returned google.longrunning.Operation, the index will be removed. During index deletion, the process could result in an error, in which case the index will move to the ERROR state. The process can be recovered by fixing the data that caused the error, followed by calling delete again.