Remove gitlab_system_hook_token from the chef recipe for customers.gitlab.com

Summary

As part of gitlab-org/customers-gitlab-com#875 (closed) SyncGitlabGroupMemberWorker and all its related setup / endpoints / secrets were removed from the Subscriptions App. This is a follow up to further clean the cookbooks and chef vault.

⚠ Probable blocker

Removing these values from the cookbooks needs to be merged: https://gitlab.com/gitlab-cookbooks/cookbook-customers-gitlab-com/-/merge_requests/104