kmscon: reset ExecStart to allow override

The getty@.service unit already has an ExecStart so we cannot simply set a new
one in order to override it or we will get this error:

systemd[1]: getty@tty1.service: Service has more than one ExecStart= setting, which is only allowed for Type=oneshot services. Refusing.

Instead "reset" ExecStart by setting it to empty which is the systemd way of
doing it.

nixos/modules/services/ttys/kmscon.nix

@@ -60,6 +60,7 @@ in {
       ConditionPathExists=/dev/tty0
 
       [Service]
+      ExecStart=
       ExecStart=${pkgs.kmscon}/bin/kmscon "--vt=%I" ${cfg.extraOptions} --seats=seat0 --no-switchvt --configdir ${configDir} --login -- ${pkgs.shadow}/bin/login -p
       UtmpIdentifier=%I
       TTYPath=/dev/%I