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/signverifymessagedialog.cpp

@@ -19,7 +19,7 @@
 #include <QClipboard>
 
 SignVerifyMessageDialog::SignVerifyMessageDialog(const PlatformStyle *_platformStyle, QWidget *parent) :
-    QDialog(parent),
+    QDialog(parent, GUIUtil::dialog_flags),
     ui(new Ui::SignVerifyMessageDialog),
     model(nullptr),
     platformStyle(_platformStyle)