Don't attempt to delete index files during flush for ephemeral buckets

When ns-server performs a flush operation it also tries to delete the index files associated with the bucket. This step (deleting indexes step) looks for the index files in the bucket specific folder in the data directory. If this directory doesn't exist then the flush operation would fail (MB-24751). Currently kv_engine is creating the data directory for ephemeral buckets. But if kv_engine team decides to remove the data directory for ephemeral bucket then ns_server should be updated to not perform the "delete indexes" step for ephemeral buckets.