Remove links to Slack

[oskarkook]

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.

Examples of this:

• https://www.reddit.com/r/elixir/comments/129w7sf/how_to_join_elixir_slack/
• https://elixirforum.com/t/unable-to-join-the-slack-channel/56331
• https://www.reddit.com/r/elixir/comments/eawv49/cant_get_an_invitation_for_the_elixir_slack/

Pointing newcomers to ask questions in Slack is not great, since it sends them down a rabbit hole of trying to find out how to even get access to it.

[oskarkook]

During my search, I also found this thread: https://www.reddit.com/r/elixir/comments/123iepb/is_it_time_to_move_away_from_slack_poll/

Overall, it seems like some part of the community would like to move away from Slack anyway.

[josevalim]

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