fix(event-exposure): return 204 instead of 200 on subscription delete

[ALIIQBAL786]

Summary

HandleDeleteAMFEventSubscription returns 200 with a null body on a successful delete. Per 3GPP TS 29.518, Namf_EventExposure's Unsubscribe operation should return 204 No Content.

This isn't just a spec nitpick - it actually breaks the standards-compliant generated SBI client in free5gc/openapi's amf/EventExposure package. That client's DeleteSubscription only special-cases 204 as success:

switch localVarHTTPResponse.StatusCode {
case 204:
    return &localVarReturnValue, nil
...

Any other status code (including the 200 this handler returns) falls through to the generic error path, so every consumer using that client sees a successful delete as a failure.

How I found this

While building and testing a NEF feature (Monitoring Event API, free5gc/nef#31) that creates and deletes Namf_EventExposure subscriptions, against a real running AMF instance - not a mock. POST worked; DELETE consistently came back as a NEF-side 500 even though AMF's own access log showed the delete had actually succeeded. Traced it to this status code mismatch.

Fix

c.JSON(http.StatusOK, nil) → c.Status(http.StatusNoContent).

Test plan

• GOOS=linux GOARCH=amd64 go build ./... and go vet ./... pass (native darwin build fails on this machine for unrelated reasons - github.com/free5gc/sctp doesn't support darwin - not something this change touches or is affected by)
• Verified against a real running AMF: DELETE /namf-evts/v1/subscriptions/{id} now returns 204, and the NEF consumer correctly treats it as success