Remove traces of BeOS.

Remove traces of BeOS.

Commit 15abc7788e6 tolerated namespace pollution from BeOS system
headers.  Commit 44f902122 de-supported BeOS.  Since that stuff didn't
make it into the Meson build system, synchronize by removing from
configure.

Author: Thomas Munro <thomas.munro@gmail.com>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Japin Li <japinli@hotmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> (the idea, not the patch)
Discussion: https://postgr.es/m/ME3P282MB3166F9D1F71F787929C0C7E7B6312%40ME3P282MB3166.AUSP282.PROD.OUTLOOK.COM