[2432] Remove explicit template instantiation in one case

I thought that explicit type name made the code more readable, but maybe
that's not the case so I've removed it now.

It still needs to be specified in the other constructor.

src/lib/dns/rrset_collection.cc

@@ -65,7 +65,7 @@ RRsetCollection::constructHelper(T source, const isc::dns::Name& origin,
 RRsetCollection::RRsetCollection(const char* filename, const Name& origin,
                                  const RRClass& rrclass)
 {
-    constructHelper<const char*>(filename, origin, rrclass);
+    constructHelper(filename, origin, rrclass);
 }
 
 RRsetCollection::RRsetCollection(std::istream& input_stream, const Name& origin,