nixos/systemd: remove obsolete DefaultCPUAccounting option

This option is made uncondiotional in systemd 258 [1].
Earlier, it defaulted to true on kernels newer than 4.15,
which applies to all supported nixos kernels.
This means removing the option does not change behavior.

[1] https://github.com/systemd/systemd/commit/29da53dde32a0097134f6c3ba5fb5220dd6e5423

nixos/modules/system/boot/systemd.nix

···
       type = lib.types.submodule {
         freeformType = types.attrsOf unitOption;
         options = {
-          DefaultCPUAccounting = mkOption {
-            type = types.bool;
-            default = cfg.enableCgroupAccounting;
-            defaultText = lib.literalExpression "config.systemd.enableCgroupAccounting";
-            description = "Turn on CPU usage accounting";
-          };
-
           DefaultIOAccounting = mkOption {
             type = types.bool;
             default = cfg.enableCgroupAccounting;