wallet2: do not repeatedly ask for pool txes sent to us

This lets a passive attacker with access to the network link
between node and wallet perform traffic analysis to deduce
when an idle wallet receives a transaction.

Reported by Tramèr et al.

src/wallet/wallet2.cpp

@@ -2968,11 +2968,6 @@ void wallet2::update_pool_state(bool refreshed)
         LOG_PRINT_L1("We sent that one");
       }
     }
-    else
-    {
-      LOG_PRINT_L1("Already saw that one, it's for us");
-      txids.push_back({txid, true});
-    }
   }
 
   // get those txes