Gui: Fix using context-menu of ExpressionLineEdit

If for the DlgExpressionInput the user config parameter 'NoSystemBsckground' is set to True it's not used as a normal dialog but as a pop-up window.
In this mode the context-menu of the ExpressionLineEdit isn't working. A previous commit (a3036d16f1) fixed a crash but not this issue.

For Qt4 DlgExpressionInput has overridden the eventFilter() to handle using the context-menu but this is not working any more for Qt5 or Qt6.
Luckily, it has appeared that with Qt5 and Qt6 it works out-of-the box now and the event filter is not needed any more.

src/Gui/DlgExpressionInput.h

@@ -76,13 +76,10 @@ public:
     QPoint expressionPosition() const;
     void   setExpressionInputSize(int width, int height);
 
-    bool eventFilter(QObject *obj, QEvent *event) override;
-
 public Q_SLOTS:
     void show();
 
 protected:
-    void showEvent(QShowEvent*) override;
     void mouseReleaseEvent(QMouseEvent*) override;
     void mousePressEvent(QMouseEvent*) override;