databases/rrdtool: Make groff a conditional dependency

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