extlinux-conf-builder: Choose whether to use DTBs on per-generation basis

Previously, the value from stdenv.platform.kernelDTB was used. That
doesn't work well if both kinds (DTB and non-DTB) of generations exist
in the system profile.

nixos/modules/system/boot/loader/generic-extlinux-compatible/extlinux-conf-builder.nix

@@ -5,5 +5,4 @@ pkgs.substituteAll {
   isExecutable = true;
   path = [pkgs.coreutils pkgs.gnused pkgs.gnugrep];
   inherit (pkgs) bash;
-  kernelDTB = pkgs.stdenv.platform.kernelDTB or false;
 }