Remove all Group Managed Accounts references

With https://gitlab.com/gitlab-com/Product/-/issues/2368 (internal) completed, we can remove all GMA references from Support.

1. Handbook: gitlab-com/www-gitlab-com!106026 (merged)
2. ZD org notes: https://gitlab.com/gitlab-com/support/support-ops/zendesk-global/organizations/-/merge_requests/34