systemd: remove hard-coded pid file in xendriverdomain service

Per the discussion in [0], the hard-coded pid file can be removed
completely. Systemd has no trouble figuring out the pid of devd all by
itself.

[0]: https://lists.xen.org/archives/html/xen-devel/2016-07/msg01393.html

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/hotplug/Linux/systemd/xendriverdomain.service.in

@@ -6,8 +6,7 @@ ConditionVirtualization=xen
 
 [Service]
 Type=forking
-ExecStart=@sbindir@/xl devd --pidfile=/var/run/xldevd.pid
-PIDFile=/var/run/xldevd.pid
+ExecStart=@sbindir@/xl devd
 
 [Install]
 WantedBy=multi-user.target