Revert isdelegation() to return boolean value again

Revert isdelegation() to return boolean value again

The isdelegation() was changed to return an isc_result_t because the
idea was to have a separate return value DNS_R_NSEC3ITERRANGE to signal
to the caller we could not verify the proof because of too many
iterations in the NSEC3 record, or perhaps ISC_R_UNEXPECTED for a more
generic cause that verification was not done.

But this would make error handling more fragile and all we care about
is whether we can reliably say the NS bit was not set.

If we can not reliably say so, we have to treat it as an insecure
referrral.

Since the answer is either yes or no, we can revert back to returning
a boolean value.