fix: usr: Revert NSEC3 closest encloser lookup improvements

The performance improvements for NSEC3 closest encloser lookups that
were restored in BIND 9.20.8 turned out to cause incorrect NSEC3 records
to be returned in nonexistence proofs and were therefore reverted again.

Closes #5292

Merge branch '5292-revert-nsec3-closest-encloser-lookup-improvements' into 'bind-9.20'

See merge request isc-projects/bind9!10443