[EE] Remove the User#is_admin? method
EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10520
Removes additional EE-specific uses of User#is_admin?
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10520
Removes additional EE-specific uses of User#is_admin?