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 <[email protected]>
Reviewed-by: Peter Eisentraut <[email protected]>
Reviewed-by: Heikki Linnakangas <[email protected]>
Reviewed-by: Japin Li <[email protected]>
Reviewed-by: Tom Lane <[email protected]> (the idea, not the patch)
Discussion: https://postgr.es/m/ME3P282MB3166F9D1F71F787929C0C7E7B6312%40ME3P282MB3166.AUSP282.PROD.OUTLOOK.COM