various: remove left-over rtc_cmos rootModule

[wolfgangwalther]

This is not needed anymore after
65a50ea.

As discussed in #359309 (comment).

@Ma27

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/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 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.

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[ElvishJerricco]

Anything we can do to test this doesn't break anything?

[wolfgangwalther]

I did the tests described in #359309 (comment) at the bottom.

[philiptaron]

I also built the tests listed.

If there is a likely break in production, it'll come from changing nixos/modules/system/boot/kernel.nix, I bet. But we ought to jiggle these defaults from time to time just to see what falls out, as a matter of not ossifying things, and this is as well-thought-out a change as I think we can hope for.