Why on earth no unset for /ip dhcp-server network?!?
This command do not work at all:
/ip dhcp-server network unset 1 next-server
I cannot answer the question why the unset command as such is not available for /ip dhcp-server network (as well as for multiple other configuration tables), but I can suggest you how to unset the next-server on a row without removing and re-creating the row as a whole:
/ip dhcp-server network set $row next-server=0.0.0.0