fix(coreos): remove refs to obsolete update-engine-reboot-manager

This was removed from CoreOS a while ago, so our unit file was having
no effect.
coreos/update_engine#28

Author: mboersma

contrib/coreos/systemd/stop-reboot-manager.service

@@ -46,16 +46,6 @@ coreos:
       [Service]
       Environment=FLEET_PUBLIC_IP=$private_ipv4
       ExecStart=/usr/bin/fleet
-  - name: stop-reboot-manager.service
-    command: start
-    content: |
-      [Unit]
-      Description=stop update-engine-reboot-manager
-
-      [Service]
-      Type=oneshot
-      ExecStart=/usr/bin/systemctl stop update-engine-reboot-manager.service
-      ExecStartPost=/usr/bin/systemctl mask update-engine-reboot-manager.service
   - name: stop-update-engine.service
     command: start
     content: |