sci-visualization/gnuplot: Drop free-noncomm license.

sci-visualization/gnuplot: Drop free-noncomm license.

File src/bitmap.c used to have a noncommercial restriction for the
raster plotting routines, until version 4.6:

   ** You may use this code for anything you like as long as
   ** you are not selling it and the credit is given and
   ** this message retained.

This was lifted in version 5.0:

   ** Jyrki Yli-Nokari, December 2013.
   ** Change to dual licence (Gnuplot + BSD) and remove previous
   ** restriction to noncommercial use.

Bug: https://bugs.gentoo.org/401191#c9
Package-Manager: Portage-2.3.80, Repoman-2.3.19
Signed-off-by: Ulrich Müller <ulm@gentoo.org>