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.