bugfix: bmc: update service requires mode

Systemd startunit requires a job-mode.  Default it to "replace."  In a
later patchset, the update systemd and verify system will collapse into
one object, but this fix applies to the current implementation.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: Ia6465abac9979501ea7149933394d8c452b2f315
diff --git a/bmc/update_systemd.cpp b/bmc/update_systemd.cpp
index d8f2329..92c97c8 100644
--- a/bmc/update_systemd.cpp
+++ b/bmc/update_systemd.cpp
@@ -45,11 +45,7 @@
     auto method = bus.new_method_call(systemdService, systemdRoot,
                                       systemdInterface, "StartUnit");
     method.append(target);
-
-    if (!mode.empty())
-    {
-        method.append(mode);
-    }
+    method.append(mode);
 
     try
     {