Previously `SystemHookPushWorker` would always be called after a push event,
and this would queue a Sidekiq job regardless of whether any system hooks
needed that event. Moreover, another call inside `Project#execute_hooks` would
also fire system hooks if they existed.

This change both removes the duplicate system hook calls. For installations
without system hooks for push events, this change also can save significant
amount of RAM used by Redis.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/50549