scripted-diff: Remove unused "What's This" button in dialogs on Windows

-BEGIN VERIFY SCRIPT-
git grep -l 'QDialog(parent)' -- src/qt | xargs sed -i -E 's/QDialog\(parent\)/QDialog\(parent, GUIUtil::dialog_flags\)/g'
-END VERIFY SCRIPT-

src/qt/utilitydialog.cpp

@@ -29,7 +29,7 @@
 
 /** "Help message" or "About" dialog box */
 HelpMessageDialog::HelpMessageDialog(QWidget *parent, bool about) :
-    QDialog(parent),
+    QDialog(parent, GUIUtil::dialog_flags),
     ui(new Ui::HelpMessageDialog)
 {
     ui->setupUi(this);