Remove event email settings for one context and mode

Permanently removes the event email settings record for the specified context and mode. Both context and mode are matched against a case-sensitive route pattern — other casings return 404 at the routing layer before this operation executes. If the deleted mode was the only configured mode for its context, the parent context block is also removed. This operation is idempotent — it returns 204 whether or not a matching record existed before the call. To disable event emails without deleting configuration, set the relevant type entries to enabled: false via POST /{context}/{mode} instead. This operation has no request body and returns no response body.