Delete grouping from work center capacity not possible

All,

The work center has capacity defined through Reference available capacity with grouping C4. We want to delete the ref.capacity and create the capacity at work center level.

When we delete the reference capacity a new grouping appears C3 and we can't delete it (gray cell).

C3 must to be deleted too but is not possible.

Any help is appreciate.

Thank you for your time!

Regards,Marius