Drop PLOT option.

gnuplot completely removed support for the gnugraph and unixplot terminal
drivers in 2011. From the one of the ChangeLog files:

2011-05-31 Ethan A Merritt <merritt@u.washington.edu>
[...]
Remove obsolete terminal drivers gnugraph and unixplot. The underlying
gnu plotutils library is itself more than 10 years out of date (last
release July 2000) and the terminal drivers haven't been upgraded for
longer than that. In any event, current gnuplot has better native
terminal drivers for the devices supported by unixplot/gnugraph.

In practice, this means the PLOT option is a no-op and the dependency on
graphics/plotutils is unnecessary. Indeed, if one looks at the port's build
logs in the cluster the following can be seen:

configure: WARNING: unrecognized options: --without-lisp-files, --with-plot,
--disable-thin-splines, --with-wx-config

PR: 205488
Approved by: glewis (maintainer)