gh-74940: Allow fallback to UTF-8 encoding on systems with no locales installed (GH-14925)

This change removes the alias of the 'C' locale to 'en_US'. Because of
this alias, it is currently impossible for an application to use
setlocale() to specify a UTF-8 locale on a system that has no locales
installed, but which supports the C.UTF-8 locale/encoding.