src.opts.mk: Retire BSD_CPIO option

[emaste]

Summary

This option was added when we introduced the BSD licensed cpio based on
libarchive so that we could install GNU cpio, libarchive cpio, or both.
GNU cpio was removed long ago and there is no longer a reason to keep
this option.

[emaste]

(src.conf.5 to be regenerated)

[emaste]

Will update commit message to make it clear we are installing bsd cpio unconditionally, not removing the option to install bsd cpio.