Remove the defer for MR refresh web hook worker

What does this MR do and why?

This MR temporarily removes the defer_on_database_health_signal configuration from the MergeRequests::Refresh::WebHooksWorker to resolve Prometheus client rate limiting errors.

When the webhook worker is deferred due to database health signals (autovacuum activity), it causes excessive concurrent requests to the Prometheus client, resulting in 429 - too many outstanding requests errors. Removing the deferral mechanism prevents this issue while the worker is being deprecated in favor of an event-based approach.

There is a discussion in #571999 to properly resolve this and add the defer back in.