[wallet] Remove return value from CommitTransaction()

CommitTransaction returns a bool to indicate success, but since commit
b3a74100b8 it only returns true, even if the transaction was not
successfully broadcast. This commit changes CommitTransaction() to return
void.

All dead code in `if (!CommitTransaction())` branches has been removed.

src/interfaces/wallet.h

@@ -141,10 +141,9 @@ public:
         std::string& fail_reason) = 0;
 
     //! Commit transaction.
-    virtual bool commitTransaction(CTransactionRef tx,
+    virtual void commitTransaction(CTransactionRef tx,
         WalletValueMap value_map,
-        WalletOrderForm order_form,
-        std::string& reject_reason) = 0;
+        WalletOrderForm order_form) = 0;
 
     //! Return whether transaction can be abandoned.
     virtual bool transactionCanBeAbandoned(const uint256& txid) = 0;