Group counter & used in maintenance item(s): Deletion not possible

What causes this issue?

You tried to delete a group counter (GC) which is used in one or more
<GL:maintenance_item>maintenance items</>.

System Response

INCLUDE 'KEINE-FUNKTION' OBJECT DOKU ID TX LANGUAGE E

How to fix this error?

If you want to delete this group counter, proceed as follows:
Check in which maintenance items the group counter is used.
Then decide whether you really want to delete the task list group
counter. If so, proceed with step 3.
Delete the reference entries to the task list group counter in the
individual maintenance items.
Delete the task list group counter.

Error message extract from SAP system. Copyright SAP SE.