[Qt] simplify ban list signal handling

- remove banListChanged signal from client model
- directly call clientModel->getBanTableModel()->refresh() without the way
  over clientModel->updateBanlist()

- also fix clearing peer detail window, when selecting (clicking)
  peers in the ban list

src/qt/clientmodel.h

@@ -91,7 +91,6 @@ Q_SIGNALS:
     void numBlocksChanged(int count, const QDateTime& blockDate);
     void alertsChanged(const QString &warnings);
     void bytesChanged(quint64 totalBytesIn, quint64 totalBytesOut);
-    void banListChanged();
 
     //! Fired when a message should be reported to the user
     void message(const QString &title, const QString &message, unsigned int style);