fix: dev: remove deadcode in `query_addbestns()`

The local variable `zfname` was released in the cleanup part of the
function if not NULL, but it turns out it is now always NULL at that
point.

The flow can get to that part only in two cases: either `zfname` is not
NULL, and then it's ownership is moved to a different variable (thus, it
is now NULL), or `zfname` is already NULL.

Removing the bit of deadcode releasing it.

Merge branch 'colin/fix-getbestns-deadcode' into 'main'

See merge request isc-projects/bind9!11790