xfig_plot_conf() is now the only function for plotting with xfig

(i.e., xfig_plot_contours() was removed as it was redundand)

All functionalities of the old xfig_plot_contours() is contained in
xfig_plot_conf(). Note that the qualifier for setting the color is
"color" (like in all xfig routines) and not "col".