fix: dev: Remove sun_path field from isc_netaddr

The `sun_path` field is not used anymore, and consumes over a hundred bytes for every `isc_netaddr_t` object. Remove it.

As `isc_netaddr_t` is used in `cfg_obj_t`, in some huge configuration trees (e.g., a million zones), the gain is almost 1GB of resident memory.

Merge branch 'colin/remove-sun-path' into 'main'

See merge request isc-projects/bind9!11184