Fix double-free of DNS_Mgr_Request object (addresses #661)

In DNS::Resolve, they could be deleted once from where they were
stored in the nb_dns_info cookie and once again from where they
were stored in the DNS_Mgr::requests list.  Before commit
bd9c937236, they were only deleted
from the requests list, so this commit reverts to that behavior
without any leaks being reported by the core/leaks tests.

src/DNS_Mgr.cc

@@ -595,8 +595,6 @@ void DNS_Mgr::Resolve()
 				}
 			else
 				--num_pending;
-
-			delete dr;
 			}
 		}