groff is required by rrdtool to format documentation, which
is only required when documentation is being built.
Accordingly, make groff conditional on the existing DOCS option.
This precludes building a litany of dependencies, mostly due
to groff requiring ghostscript.
While here, remove unneeded post-patch in the case the DOCS option is
switched off.
Submitted by: koobs
Differential Revision: https://reviews.freebsd.org/D14732
b80e953f45