Remove listing for broken icecast plugin. (#7408)
Remove listing for broken icecast plugin. (#7408)

The repo description for the [3rd party Icecast plugin](https://github.com/e00E/lets-encrypt-icecast) says that the plugin isn't currently working and the repository hasn't been updated since 2017. Since it seems broken and unmaintained, let's remove it from the list of third party plugins.

I would happily add it again to the list of third party plugins if people fix and maintain it.
stweil/letsencrypt,lmcro/letsencrypt,letsencrypt/letsencrypt,stweil/letsencrypt,lmcro/letsencrypt,letsencrypt/letsencrypt