rem: usr: Remove NZF file support in favor of NZD (New Zone Database)

The NZF (New Zone File) backend for storing rndc addzone configurations
has been removed; LMDB-based NZD is now the only storage backend and
LMDB is now a required build dependency.

Existing NZF files are automatically migrated to NZD on startup, so no manual
intervention is required when upgrading.

Merge branch 'ondrej/drop-nzf-support' into 'main'

See merge request isc-projects/bind9!11688