Why you can't find any delete actions for the DocuRef table in AX7

There aren't any...

In AX7 clean up of records in DocuRef is handled by a recurring batch job. The batch job is automatically created during AX initialization when you deploy a new solution. The batch job is listed as "Scan for orphaned document references" and it is scheduled to run every sixth hour.

I can't find the job between the batch jobs in the demo data though. But you can add it by calling the loadData method of the class DocumentManagementSetup.