chg: dev: Delete cache rdatasets directly instead of tombstoning them

Deleting an rdataset from the cache left a placeholder entry
behind that every lookup had to skip until it aged out. Deletion
now removes the entry outright, and the placeholder mechanism is gone.

Merge branch 'ondrej/qpcache-cleanup-delete-tombstone' into 'main'

See merge request isc-projects/bind9!12454