mash: Disable ash_sysui; begin removal.

mash/session/session.cc

Index: mash/session/session.cc
diff --git a/mash/session/session.cc b/mash/session/session.cc
index 8b137576a2d7d469645e871f51779d13ae881e3e..ac17014df0a72ea309a7ca225cde2e8963494857 100644
--- a/mash/session/session.cc
+++ b/mash/session/session.cc
@@ -30,7 +30,6 @@ Session::~Session() {}
 void Session::OnStart(const shell::Identity& identity) {
   StartAppDriver();
   StartWindowManager();
-  StartSystemUI();
   StartQuickLaunch();
 }
 
@@ -95,12 +94,6 @@ void Session::StartWindowManager() {
                  base::Unretained(this)));
 }
 
-void Session::StartSystemUI() {
-  StartRestartableService("mojo:ash_sysui",
-                          base::Bind(&Session::StartSystemUI,
-                                     base::Unretained(this)));
-}
-
 void Session::StartAppDriver() {
   StartRestartableService(
       "mojo:app_driver",