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/msw/control.cpp

@@ -410,7 +410,7 @@ wxMSWOwnerDrawnButtonBase::MSWMakeOwnerDrawnIfNecessary(const wxColour& colFg)
 {
     // The only way to change the checkbox foreground colour when using
     // themes is to owner draw it.
-    if ( wxUxThemeEngine::GetIfActive() )
+    if ( wxUxThemeIsActive() )
         MSWMakeOwnerDrawn(colFg.IsOk());
 }