[trac80] cleanup: replaced class-dependent find for rrsetlist with class independent one.

In this case, the behavior should be the same because the class is used
from the data source, not from the query.  This change is mainly for
consistency (but as a bonus we can get rid of an unused temporary variable).

src/lib/datasrc/data_source.cc

@@ -956,9 +956,8 @@ DataSrc::doQuery(Query& q) {
                     // Add the NS records for the enclosing zone to
                     // the authority section.
                     RRsetList auth;
-                    const DataSrc* ds = zoneinfo.getDataSource();
-                    if (!refQuery(q, Name(*zonename), zoneinfo, auth)  ||
-                        !auth.findRRset(RRType::NS(), ds->getClass())) {
+                    if (!refQuery(q, Name(*zonename), zoneinfo, auth) ||
+                        !findRRsetFromList(auth, RRType::NS())) {
                         isc_throw(DataSourceError,
                                   "NS RR not found in " << *zonename << "/" <<
                                   q.qclass());