Motivation for this change

The last use of kernelMajor in Nixpkgs was removed in 2018.

Even then, I'm not positive it was actually in an exercised code path.

AFAIUI this is now totally redundant and useless as it really was meant
for the 2.4 -> 2.6 transition.

Context:

• https://logs.nix.samueldr.com/nixos/2020-11-23#1606100144-1606100762;

If this is still in use, educate us :). I think it's simply that all parts using this were removed one by one, and we're left with this now-unused option.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.