nixos/powerManagement: remove duplicate definition

When not set just use the kernel default. `nixos-generate-config` will pick
a reasonable default.

cc #34350

nixos/modules/config/power-management.nix

···
 
   config = mkIf cfg.enable {
 
-    # Leftover for old setups, should be set by nixos-generate-config now
-    powerManagement.cpuFreqGovernor = mkOptionDefault "ondemand";
-
     systemd.targets.post-resume = {
       description = "Post-Resume Actions";
       requires = [ "post-resume.service" ];