share/doc: Remove documents that we haven't been installing for ages.

share/doc: Remove documents that we haven't been installing for ages.

While all of those are old and some of them are quite outdated, some
others are actually still useful and have information that is not
contained in any manual page (such as information on the internals
of config(8), for example). But since we haven't been installing them
since 15 years now (see ce233c472eb2cc66b348daa45965ffeb85ad7fe1) and
they are archived elsewhere and easily findable on the web, remove
them from our tree.

Also, they would need groff in base to build, which is no longer the
case in DragonFly since 1419a4e1759b9901ee3a1d147d0f7bee0394d6f3.

See for example https://docs.freebsd.org/44doc/ for FreeBSD's
archive.