gimp: remove 2.6, because it's old and vulnerable

CVE-2012-2763 and three others. If we really need to resurrect it,
I think I saw debian could have patches for these somewhere.

Also add linux platforms for 2.8.

pkgs/applications/graphics/gimp/2.8.nix

@@ -34,5 +34,6 @@ stdenv.mkDerivation rec {
     description = "The GNU Image Manipulation Program";
     homepage = http://www.gimp.org/;
     license = "GPL";
+    platforms = stdenv.lib.platforms.linux;
   };
 }

pkgs/applications/graphics/gimp/default.nix

@@ -1,32 +0,0 @@
-{ stdenv, fetchurl, pkgconfig, gtk, freetype
-, fontconfig, libart_lgpl, libtiff, libjpeg, libpng, libexif, zlib, perl
-, perlXMLParser, python, pygtk, gettext, xlibs, intltool, babl_0_0_22, gegl_0_0_22
-}:
-
-stdenv.mkDerivation rec {
-  name = "gimp-2.6.12";
-
-  src = fetchurl {
-    url = "ftp://ftp.gtk.org/pub/gimp/v2.6/${name}.tar.bz2";
-    sha256 = "0qpcgaa4pdqqhyyy8vjvzfflxgsrrs25zk79gixzlnbzq3qwjlym";
-  };
-
-  buildInputs = [
-    pkgconfig gtk freetype fontconfig
-    libart_lgpl libtiff libjpeg libpng libexif zlib perl
-    perlXMLParser python pygtk gettext intltool babl_0_0_22 gegl_0_0_22
-  ];
-
-  passthru = { inherit gtk; }; # probably its a good idea to use the same gtk in plugins ?
-
-  configureFlags = [ "--disable-print" ];
-
-  # "screenshot" needs this.
-  NIX_LDFLAGS = "-rpath ${xlibs.libX11}/lib";
-
-  meta = {
-    description = "The GNU Image Manipulation Program";
-    homepage = http://www.gimp.org/;
-    license = "GPL";
-  };
-}

pkgs/top-level/all-packages.nix

@@ -7928,11 +7928,6 @@ let
 
   get_iplayer = callPackage ../applications/misc/get_iplayer {};
 
-  gimp_2_6 = callPackage ../applications/graphics/gimp {
-    inherit (gnome) libart_lgpl;
-    libpng = libpng12;
-  };
-
   gimp_2_8 = callPackage ../applications/graphics/gimp/2.8.nix {
     inherit (gnome) libart_lgpl;
     webkit = null;