lxc/completion: Don't hide default completion command (#14474)

The completion command is a useful tool that includes detailed
instructions for setting up completions across shell environments. Most
CLI applications don't hide this by default so it's quite intuitive for
a user to locate/use this command. Furthermore, the LXD snap completions
are only set up for Bash shells.

With this change, running `lxc completion <shell> --help` shows
instructions on generating completions.

lxc/main.go

@@ -87,7 +87,6 @@ All of LXD's features can be driven through the various commands below.
 For help with any of those, simply call them with --help.`))
 	app.SilenceUsage = true
 	app.SilenceErrors = true
-	app.CompletionOptions = cobra.CompletionOptions{HiddenDefaultCmd: true}
 
 	// Global flags
 	globalCmd := cmdGlobal{cmd: app, asker: cli.NewAsker(bufio.NewReader(os.Stdin), nil)}