Remove custom final_message

At this point all of the custom final_message is now obsolete.
Remove it, letting us instead use the default final_message.
Leave a note about the above.

live-build/ubuntu-server/includes.binary/overlay/etc/cloud/cloud.cfg

@@ -32,7 +32,7 @@ network:
                 name: "eth*"
             dhcp4: true
 
-final_message: "## template: jinja\nCloud-init v. {{version}} finished at {{timestamp}}. Datasource {{datasource}}.  Up {{uptime}} seconds\n\n\nWelcome to Ubuntu Server Installer!\n\nIf you provided SSH keys in the cloud-init datasource, they were also provisioned to the installer user.\n\nIf you have access to the graphical console, like TTY1 or HMC ASCII terminal you can complete the installation there too."
+# We used to have a custom final_message here.  Just use the default instead.
 
 # Example datasource config
 # datasource: