remove Index field from EmitNodeEventsResponse

`Index` is already included as part of `WriteMeta` embedding.

This is a backward compatible change: Clients never read the field; and
Server refernces to `EmitNodeEventsResponse.Index` would be using the
value in `WriteMeta`, which is consistent with other response structs.

nomad/structs/structs.go

@@ -1282,7 +1282,6 @@ type EmitNodeEventsRequest struct {
 // EmitNodeEventsResponse is a response to the client about the status of
 // the node event source update.
 type EmitNodeEventsResponse struct {
-	Index uint64
 	WriteMeta
 }