OutputPane: Use setter for display name

Change-Id: I2e89fbd052330c9e7124272c2f2fa81a0db94de9
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

src/plugins/coreplugin/messageoutputwindow.h

@@ -20,7 +20,6 @@ public:
 
     QWidget *outputWidget(QWidget *parent) override;
 
-    QString displayName() const override;
     int priorityInStatusBar() const override;
     void clearContents() override;