dns_adb: Remove deadnames and deadentries

dns_adb: Remove deadnames and deadentries

Previously, the name and entry buckets were much larger, so the dead
names and entries were moved to a secondary list to be cleaned
later (f.e. after the already running fetch has been canceled).  After
the last refactoring, the bucket now contains only the name (entry)
itself and thus the extra list has a little use.  Remove the .deadnames
and .deadentries from dns_adbnamebucket_t and dns_adbentrybucket_t
structures.