[WIP] Deprecated/remove flexible entity

| Q                   | A
| ------------------- | ---
| Bug fix?            | N (remove deprecated bundle)
| New feature?        | N
| BC breaks?          | Y
| Tests pass?         | ?
| Scenarios pass?     | ?
| Checkstyle issues?  | N
| Changelog updated?  | Y
| Fixed tickets       |
| Doc PR              |