xanmod-kernels: enable writeback throttling by default

follows commit https://gitlab.com/xanmod/linux/-/commit/c4933254169ad0cfc9164ed262361b9d50439e06

pkgs/os-specific/linux/kernel/xanmod-kernels.nix

···
       HZ = freeform "250";
       HZ_250 = yes;
       HZ_1000 = no;
-
-      # Disable writeback throttling by default
-      BLK_WBT_MQ = lib.mkOverride 60 no;
     };
 
     extraMeta = {