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 gnu plotutils library is itself more than 10 years out of date (last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)

math/gnuplot/Makefile

@@ -2,6 +2,7 @@
 
 PORTNAME=	gnuplot
 PORTVERSION=	5.0.1
+PORTREVISION=	1
 CATEGORIES=	math graphics
 MASTER_SITES=	SF
 
@@ -30,13 +31,12 @@ PORTEXAMPLES=	*
 
 # MAKE_JOBS_UNSAFE=yes
 
-OPTIONS_DEFINE=	CAIRO DOCS EXAMPLES GD GRIDBOX PDF PLOT THINSPLINES WX X11
+OPTIONS_DEFINE=	CAIRO DOCS EXAMPLES GD GRIDBOX PDF THINSPLINES WX X11
 GRIDBOX_DESC=		Use the gridbox optimization for hidden3d
-PLOT_DESC=		Enable plot support
 THINSPLINES_DESC=	Enable thin plate splines for grids in dgrid3d
 WX_DESC=		wxWidgets (formerly wxWindows) support
 
-OPTIONS_DEFAULT=CAIRO GD PLOT WX X11
+OPTIONS_DEFAULT=CAIRO GD WX X11
 OPTIONS_SUB=	yes
 
 CAIRO_LIB_DEPENDS=	libcairo.so:${PORTSDIR}/graphics/cairo
@@ -53,11 +53,6 @@ PDF_LIB_DEPENDS=	libpdf.so:${PORTSDIR}/print/pdflib
 PDF_CONFIGURE_ON=	--with-pdf=${LOCALBASE}
 PDF_CONFIGURE_OFF=	--without-pdf
 
-PLOT_USE=		XORG=xaw,xmu,xt,xext
-PLOT_LIB_DEPENDS=	libplot.so:${PORTSDIR}/graphics/plotutils
-PLOT_CONFIGURE_ON=	--with-plot=${LOCALBASE}
-PLOT_CONFIGURE_OFF=	--without-plot
-
 THINSPLINES_CONFIGURE_ENABLE=	thin-splines
 
 X11_USE=	XORG=x11