hooks: Remove update feature flag
The update hooks was enabled by default, and as such is being used by customers and GitLab.com in production. The feature is stable, and a nice performance improvement. This change removes the feature gate, and removes the option to disable the feature.
The biggest change are the cascading effects of removing a second implemenation. This change does not yet remove the old hook as there might be a race condition during deploy where the old hook is executed while the file is removed. In such a case, the hook wouldn't be executed. The old hook can be removed one release later.