Prevent users with admin_group_member custom ab. to manage custom roles

Merge branch 'security-fix-cr-edit-17-2' into '17-2-stable-ee'

See merge request gitlab-org/security/gitlab!4426

Changelog: security

ee/app/policies/ee/group_policy.rb

@@ -544,10 +544,6 @@ module GroupPolicy
         enable :admin_member_role
       end
-      rule { custom_roles_allowed & can?(:admin_group_member) }.policy do
-        enable :admin_member_role
-      end
-
       rule { custom_role_enables_admin_cicd_variables }.policy do
         enable :admin_cicd_variables
       end

ee/spec/policies/group_policy_spec.rb

@@ -3582,7 +3582,7 @@ def create_member_role(member, abilities = member_role_abilities)
     context 'for a member role with admin_group_member true' do
       let(:member_role_abilities) { { admin_group_member: true } }
-      let(:allowed_abilities) { [:admin_group_member, :admin_member_role] }
+      let(:allowed_abilities) { [:admin_group_member] }
       it_behaves_like 'custom roles abilities'