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.

docs/changes.txt

@@ -48,10 +48,11 @@ Changes in behaviour which may result in build errors
 - "webview" library is not included in `wx-config --libs` output any more, you
   need to request it explicitly, e.g. `wx-config --libs std,webview`.
 
-- wxMSW now requires linking with shlwapi.lib and version.lib. This is done
-  automatically in most cases, but if you use a static build of the library
-  with a non-MSVC compiler such as MinGW and do not use wx-config, then you
-  will need to add these libraries to your make or project files yourself.
+- wxMSW now requires linking with uxtheme.lib, shlwapi.lib and version.lib.
+  This is done automatically in most cases, but if you use a static build of
+  the library with a non-MSVC compiler such as MinGW and do not use wx-config,
+  then you will need to add these libraries to your make or project files
+  yourself.
 
 - WXWIN_OS_DESCRIPTION doesn't exist any longer, use wxGetOsDescription().