PIM-2778: Fix flash message on attribute group removal

| Q                    | A
| -------------------- | ---
| Bug fix?             | N
| New feature?         | N
| BC breaks?           | N
| CI currently passes? | Y
| Tests pass?          | Y
| Scenarios pass?      | running
| Checkstyle issues?*  | N
| PMD issues?**        | N
| Changelog updated?   | -
| Fixed tickets        | -
| Doc PR               | -