units: drop KillMode= from initrd-switch-root.service

The service already has DefaultDeps disabled, so systemd should not try to stop
it. And if it *does* get stopped, we don't want the zombie process around.
KillMode=none does not change anything in the killall() phase, and we already
use argv[0][0] = '@' to protect against that anyway. KillMode=none should not
be useful in normal operation, so let's leave it out.

units/initrd-switch-root.service.in

@@ -16,4 +16,3 @@ AllowIsolate=yes
 [Service]
 Type=oneshot
 ExecStart=@rootbindir@/systemctl --no-block switch-root /sysroot
-KillMode=none