User is not removed from group when they are removed from LDAP.

• Zendesk issue: https://gitlab.zendesk.com/agent/tickets/12933
• GitLab version: 8.2.2

If LDAP group sync is enabled and a user account is completely removed from LDAP, they will not be removed from the GitLab group. The sync worker correctly blocks the user on the next sync, but they are still a member of the group. This isn't a security concern because the user is already blocked, but we should also remove them from the synced groups to avoid confusion.