refactor(icon): replaced non-existing `x_placeholder` by `db` for the moment

[mfranzke]

Proposed changes

Resolves https://github.com/db-ui/mono/issues/3266

Types of changes

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Refactoring (fix on existing components or architectural decisions)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation Update (if none of the other choices apply)

Further comments