Simplify the code and the server setup by deleting the pseudo-manpath

Simplify the code and the server setup by deleting the pseudo-manpath
"mandoc" that was used for man.cgi(8) documentation and by assuming
that the apropos(1) and man.cgi(8) manuals are simply installed in
the default manpath.  Even though man.cgi(8) is not installed by
default when installing OpenBSD, it is easy to copy it into the
default manpath used for man.cgi(8).

Idea found when considering a question asked by wrant dot com.