fix: dev: Clean up 'nodetach' in ns_client

The 'nodetach' member is a leftover from the times when non-zero
'stale-answer-client-timeout' values were supported, and currently
is always 'false'. Clean up the member and its usage.

Merge branch 'aram/cleanup-ns-client-nodetach' into 'main'

See merge request isc-projects/bind9!9592