Remove configure check for _configthreadlocale().

Remove configure check for _configthreadlocale().

All modern Windows systems have _configthreadlocale().  It was first
introduced in msvcr80.dll from Visual Studio 2005.  Historically, MinGW
was stuck on even older msvcrt.dll, but added its own dummy
implementation of the function when using msvcrt.dll years ago anyway,
effectively rendering the configure test useless.  In practice we don't
encounter the dummy anymore because modern MinGW uses ucrt.

Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/CWZBBRR6YA8D.8EHMDRGLCKCD%40neon.tech