openapi: Remove add operation from op list of stream event.

This removes the `add` from op list of stream event, as we do not
actually generate the stream/add event in the API, and when a stream
is created we identify it using the `create` operation.

(This was likely just a mistake introduced as a result of the fact
that `create` does not fit the normal naming scheme; probably
long-term we should actually migrate this to "add", but more important
for now is to document what's accurate).

Part of #17568.

zerver/openapi/zulip.yaml

@@ -935,7 +935,6 @@ paths:
                                 op:
                                   type: string
                                   enum:
-                                    - add
                                     - create
                                 streams:
                                   type: array