Use dotted pens in wxGenericTreeCtrl everywhere

There hopefully isn't any reason preventing us from using dotted pens 20
years after the commit 00e12320ca (Some debug code. Removed flicker form
wxTreeCtrl. Changes.txt update., 1999-12-22) disabling them because they
were "too slow under XFree86" was made.

src/generic/treectlg.cpp

@@ -1009,15 +1009,8 @@ bool wxGenericTreeCtrl::Create(wxWindow *parent,
     if (!m_hasFont)
         SetOwnFont(attr.font);
 
-    // this is a misnomer: it's called "dotted pen" but uses (default) wxPENSTYLE_SOLID
-    // style because we apparently get performance problems when using dotted
-    // pen for drawing in some ports -- but under MSW it seems to work fine
-#ifdef __WXMSW__
     m_dottedPen = wxPen(wxSystemSettings::GetColour(wxSYS_COLOUR_GRAYTEXT),
                         1, wxPENSTYLE_DOT);
-#else
-    m_dottedPen = *wxGREY_PEN;
-#endif
 
     SetInitialSize(size);