Unify how we use isc_refcount_decrement() to destroy object

Unify how we use isc_refcount_decrement() to destroy object

The isc_refcount_decrement() was either used as:

    if (isc_refcount_decrement() == 1) { destroy(); }

or

    if (isc_refcount_decrement() != 1) { return; } destroy();

This commits eradicates the last usage of the later, so the code is unified to
use the former.