In preparation for the removal of the roff toolchain, disconnect the
roff documentation from the build.

Those documents will be added to the doc tree and distributed as PDF from
the documentation website. As they are valuable has history, but do not match
current FreeBSD

Further more, the ascii format we were using to distribute them is not really
accurate for such documents.

more details:
https://lists.freebsd.org/pipermail/freebsd-arch/2017-May/018211.html