[wallet] Schedule tx rebroadcasts in wallet

Removes the now-unused Broadcast/ResendWalletTransactions interface from
validationinterface.

The wallet_resendwallettransactions.py needs a sleep added at the start
to make sure that the rebroadcast scheduler is warmed up before the next
block is mined.

src/net_processing.cpp

@@ -3546,14 +3546,6 @@ bool PeerLogicValidation::SendMessages(CNode* pto)
             }
         }
 
-        // Resend wallet transactions that haven't gotten in a block yet
-        // Except during reindex, importing and IBD, when old wallet
-        // transactions become unconfirmed and spams other nodes.
-        if (!fReindex && !fImporting && !IsInitialBlockDownload())
-        {
-            GetMainSignals().Broadcast(connman);
-        }
-
         //
         // Try sending block announcements via headers
         //