replace fonts-freefont-otf with fonts-unifont

The fonts-freefont-otf package has been included in this list, because it features some "wide Unicode coverage". However, it is quite visually unappealing otherwise (e.g. its latin glyphs are based on the same ones from the gsfonts package that the glyphs in fonts-urw-base35 are based upon).

Replace this font with fonts-unifont, which is even more visually unappealing, but at least contains glyphs for every printable code point in the Unicode BMP - which again is quite a win.