usr.bin/bc: remove OpenBSD derived bc and dc commands

In 2020, an improved implementation of the bc and dc commands
developed by Gavin D. Howard has been imported into FreeBSD.
It has replaced the OpenBSD-derived versions of these commands
in all currently supported FreeBSD releases.

The OpenBSD versions could still be built using the WITHOUT_GH_BC
option. There have been no reports of problems or unexpected
deviations from the OpenBSD version for some time, therefore
keeping the OpenBSD version is no longer required in FreeBSD.

This commit removes the option to build the OpenBSD version and
corresponding source files from -CURRENT. No MFC is planned, all
currently released FreeBSD versions should retain the build option.

The WITHOUT_GH_BC option is no longer accepted and will cause
make buildworld to fail.

Reviewed by: des, emaste
Approved by: des
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D46876