Remove platform-specific translations support

As shown by the fact that no other platform-specific translations were
ever added since the initial version of locale/msw/it.po added way back
in 4d931bcca0 (Translate '&Help' to '&' for italian Windows only,
2005-08-12), this is not especially useful, so just handle this
particular case specially in wxGetStockLabel() and remove support for
platform-specific translations.

This allows to simplify the makefiles and the catalog loading code as
there is no need to deal with the files in subdirectories any more.

src/common/translation.cpp

@@ -1522,14 +1522,6 @@ bool wxTranslations::AddStdCatalog()
     if ( !AddCatalog(wxS("wxstd")) )
         return false;
 
-    // there may be a catalog with toolkit specific overrides, it is not
-    // an error if this does not exist
-    wxString port(wxPlatformInfo::Get().GetPortIdName());
-    if ( !port.empty() )
-    {
-        AddCatalog(port.BeforeFirst(wxS('/')).MakeLower());
-    }
-
     return true;
 }