Remove unused userName config variable

Removed in favor of more flexible reading from stdin.  A different
program does the fetching.  (`unpaginate` is one option.  The format
also allows non-GitHub repos, provided the JSON objects have the
`full_name` and `git_url` fields.)

go/stars/stars.go

)
var config struct {
	userName    string
	directory   string
	concurrency int
}
func init() {
	flag.StringVar(&config.userName, "user", "heyLu", "The GitHub user to fetch stars for")
	flag.StringVar(&config.directory, "directory", "github-stars", "The directory to store the repos in")
	flag.IntVar(&config.concurrency, "concurrency", 10, "The number of repos to update concurrently")
}