nixos/systemd-nspawn: Remove dependency on bogus "machine.target"

"machine.target" doesn't actually exist, it's misspelled version
of "machines.target".  However, the "systemd-nspawn@.service"
unit already has a default dependency on "machines.target"

nixos/modules/system/boot/systemd-nspawn.nix

@@ -112,9 +112,6 @@ in {
 
       environment.etc."systemd/nspawn".source = generateUnits "nspawn" units [] [];
 
-      systemd.services."systemd-nspawn@" = {
-        wantedBy = [ "machine.target" ];
-      };
   };
 
 }