nixos/plasma5: dont set "plasma" to services.xserver.displayManager.defaultSession

[SaltyKitkat]

Description of changes

close #291501

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/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 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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[LunNova]

Not sure this is a good idea as there's now nothing to deprioritize plasma (wayland) on plasma 5 when plasma's the only DE enabled.

[K900]

What are you trying to do with this and why?

[SuperSandro2000]

From the issue:

However, since plasma wayland seems working fine today, maybe we should remove this workaround.

That isn't true. Plasma 5 Wayland is still buggy and not recommendable. Plasma 6 Wayland is in a usable state.

[SaltyKitkat]

Thanks for your attention.
I come with this idea because I found that services.xserver.displayManager.defaultSession is set to plasma(x11) as long as I enable plasma desktop. So I have to choose another session every time when sddm choose plasma(x11) as default, which is quite annoying.
After knowing

Plasma 5 Wayland is still buggy and not recommendable.

and #139657 (comment), I found this is not a good idea, and I will close this.