Remove the pmake book from the published docs.

It doesn't at all match either bmake, or the make that bmake replaced
(confusingly also sometimes referred to as pmake and fmake). The style
of the Makefiles does not bear any semblance to ours, the commands are
different, and so are the options.

It's a fascinating document, and it's good that it's not going to completely
disappear; it'll be visible in the older versions [1] as well as being
searchable, but it's really not something we should have presented as being
in any way useful documentation nowadays.

[1] https://docs.freebsd.org/doc/9.0-RELEASE/usr/share/doc/freebsd/en_US.ISO8859-1/books/pmake/book.html

PR: doc/211949
Reviewed by: linimon
Reviewed by: bcr