rem: dev: Remove unused closest encloser proof caching

BIND used to cache an NSEC3 closest encloser proof alongside positive wildcard answers so that a resolver could re-send it when answering from its cache. That stopped being used in
BIND 9.9 (2011), when positive wildcard responses were changed to omit that NSEC3 record — RFC 5155 requires only the next closer name proof — and the closest encloser came to be
derived during validation instead. The caching code has been unreachable ever since, so this removes it with no change in behaviour.

Closes #5803

Merge branch '5803-remove-addclosest' into 'main'

See merge request isc-projects/bind9!11779