Remove links to Slack (#1707)

It's not possible to join the Slack without being invited. Getting an
invite currently means that you actually have to go to one of the other
communities and then ask for an invite.

Author: oskarkook

_includes/important-links.html

@@ -23,7 +23,6 @@ <h3 class="widget-title">Join the Community</h3>
     <li><a class="spec" href="https://twitter.com/elixirlang">@elixirlang on Twitter</a></li>
     <li><a class="spec" href="irc://irc.libera.chat/elixir">#elixir on irc.libera.chat</a></li>
     <li><a class="spec" href="http://elixirforum.com">Elixir Forum</a></li>
-    <li><a class="spec" href="https://elixir-lang.slack.com/">Elixir on Slack</a></li>
     <li><a class="spec" href="https://discord.gg/elixir">Elixir on Discord</a></li>
     <li><a class="spec" href="https://www.meetup.com/topics/elixir/">Meetups around the world</a></li>
     <li><a class="spec" href="https://github.com/elixir-lang/elixir/wiki">Wiki with events and resources maintained by

install.markdown

@@ -216,7 +216,6 @@ After Elixir is up and running, it is common to have questions as you learn and
 
   * [#elixir on irc.libera.chat](irc://irc.libera.chat/elixir)
   * [Elixir Forum](http://elixirforum.com)
-  * [Elixir on Slack](https://elixir-lang.slack.com)
   * [Elixir on Discord](https://discord.gg/elixir)
   * [elixir tag on StackOverflow](https://stackoverflow.com/questions/tagged/elixir)