Don't try to center task dialogs under Windows.

This is either unnecessary or doesn't work anyhow (they are always centered on
the parent window) and just results in debug error messages.

Simply don't do anything in wxMessageDialog::Centre() when using task dialog
implementation under MSW.

Closes #12699.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66237 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/msgdlg.h

@@ -32,6 +32,10 @@ public:
     // return the font used for the text in the message box
     static wxFont GetMessageFont();
 
+protected:
+    // Override this as task dialogs are always centered on parent.
+    virtual void DoCentre(int dir);
+
 private:
     // hook procedure used to adjust the message box beyond what the standard
     // MessageBox() function can do for us