rem: usr: Remove legacy special handling for SIG, NXT, and KEY records

BIND no longer applies legacy RFC 2535 handling to the obsolete ``SIG``, ``NXT``
and ``KEY`` record types; they are now served as plain zone data. Zones with
both a ``CNAME`` and a ``KEY`` and or ``NXT`` at the same name — invalid under
:rfc:`2181` — will now fail to load and must be corrected.

Closes #6007

Merge branch '6007-remove-SIG-and-NXT-special-handling' into 'main'

See merge request isc-projects/bind9!12056