fix: dev: Null clausedefs for ancient options

This commit nulls all type fields for the clausedef lists that are
declared ancient, and removes the corresponding cfg_type_t and parsing
functions when they are found to be unused after the change.

Among others, it removes some leftovers from #1913.

Closes #4962

Merge branch '4962-null-clausedef-types-for-ancient-options' into 'main'

See merge request isc-projects/bind9!9552