Replace ad hoc throttling fix with throttled

`throttled` is a service designed to fix the same CPU throttling bug,
but it's an actively maintainted upstream project that we can use.

dell/xps/13-9370/default.nix

@@ -5,7 +5,6 @@
     ../../../common/cpu/intel/kaby-lake
     ../../../common/pc/laptop
     ../../../common/pc/laptop/acpi_call.nix
-    ../../../common/pc/laptop/cpu-throttling-bug.nix
   ];
 
   # Force S3 sleep mode. See README.wiki for details.
@@ -13,4 +12,6 @@
 
   # touchpad goes over i2c
   boot.blacklistedKernelModules = [ "psmouse" ];
+
+  services.throttled.enable = lib.mkDefault true;
 }