Use Composer's HttpDownloader and drop the dependency on Guzzle

[phenaproxima]

We use Guzzle to download TUF metadata, but Composer already comes with an HTTP downloader that should be able to do that for us, especially once php-tuf/php-tuf#290 is merged and drops support for promises. I think it would be a good idea to adopt the HTTP downloader and drop the dependency on Guzzle.