linux: remove hack for old kernels

[alyssais]

Description of changes

The oldest kernel supported in Nixpkgs is now 4.19 LTS, so we don't need this any more.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[fabianhjr]

Building 6.5.7-hardened1

[fabianhjr]

Had a conflict, there was a related change on staging that hasn't made it to staging-next nor the main branch: #245968

[alyssais]

Yeah, that's why this PR targets staging.

[fabianhjr]

Ah right, missed that and was using git am. Already passed the config phase and finishing up compilation.