qt: remove transaction count from overview page

It was needlessly confusing people, as it doesn't necessarily match the
number of transactions in the transaction list.

src/qt/overviewpage.h

@@ -30,7 +30,6 @@ public:
 
 public slots:
     void setBalance(qint64 balance, qint64 unconfirmedBalance, qint64 immatureBalance);
-    void setNumTransactions(int count);
 
 signals:
     void transactionClicked(const QModelIndex &index);