Remove MSW wxUxThemeEngine class

This undocumented "private" class was used for various windows UxTheme
functions which are available since WinXP. As wxWidgets 3.1 is XP+ it
does not make sense anymore to load the theme functions dynamically.

src/generic/combog.cpp

@@ -125,14 +125,7 @@ bool wxGenericComboCtrl::Create(wxWindow *parent,
         border = wxBORDER_SIMPLE;
 #elif defined(__WXMSW__)
     if ( !border )
-        // For XP, have 1-width custom border, for older version use sunken
-        /*if ( wxUxThemeEngine::GetIfActive() )
-        {
-            border = wxBORDER_NONE;
-            m_widthCustomBorder = 1;
-        }
-        else*/
-            border = wxBORDER_SUNKEN;
+        border = wxBORDER_SUNKEN;
 #else
 
     //