Gui: Stop remember state expression dialog

Before this commit, the expression dialog remembered the state of
whether the varset information should be shown.  This is removed
according to #17075.

src/Gui/Dialogs/DlgExpressionInput.h

@@ -118,7 +118,7 @@ private:
 
     int minimumWidth;
 
-    static bool varSetsVisible;
+    bool varSetsVisible;
     std::unique_ptr<QTreeWidget> treeWidget;
     QPushButton* okBtn = nullptr;
     QPushButton* discardBtn = nullptr;