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;
 			}
 		}
 

testing/btest/core/leaks/dns.bro

@@ -4,6 +4,10 @@
 #
 # @TEST-EXEC: HEAP_CHECK_DUMP_DIRECTORY=. HEAPCHECK=local bro -m -r $TRACES/wikipedia.trace %INPUT
 
+const foo: set[addr] = {
+     google.com
+};
+
 # Add the state tracking information variable to the connection record
 
 event connection_established(c: connection)