bsd.mkopt.mk: Avoid warning from makeman

[emaste]

Summary

Setting WITHOUT_X where X is a __REQUIRED_OPTION produces a warning.
This happened also during makeman's `make showconfig` invocatation, but
the warning serves no purpose there, and clutters makeman's output.

[emaste]

Doesn't quite work

For reference the warning we get:

............................make[1]: "/home/emaste/src/freebsd-git/wipbsd/share/mk/bsd.mkopt.mk" line 71: warning: WITHOUT_CASPER option ignored: it is no longer supported
make[1]: "/home/emaste/src/freebsd-git/wipbsd/share/mk/bsd.mkopt.mk" line 71: warning: WITHOUT_CASPER option ignored: it is no longer supported
....................................................................................................................................................................................................................................

[brooks]

Seems reasonable.