Remove GitLab group membership check

Previously, we had all developer rotate through the RM role and we had no access-requests type of controls over who is being added to groups and who can do actions.

The code removed in this MR used to:

1. Load release manager information from www-gitlab-com repository
2. Query each release-manager group for each of dev, ops, and GitLab.com
3. If a person was found in any of these groups that was not expected, the sync task would fail

After the sync task fails, it was expected for existing release managers to manually confirm whether the user should be in any of above-mentioned groups.

This is no longer needed because:

1. Delivery team is owner of release-manager duties
2. We are no longer triggering commands from local machines, but through CI and Chatops
3. config/release_managers.yml contains the list of people who are allowed to run these commands, and people outside of these names will not be able to run them
4. Only active release managers should be in release-managers groups

Closes gitlab-com/gl-infra/delivery#810 (closed)