[v3.2] Remove Slack notifications for CI failures

[waiting-for-dev]

Summary

We were storing the Slack secrets on a CircleCI context. Although we were also passing them to forks, it resulted on unauthorized builds for external contributions.

We could work around the issue in two ways:

• Having the secrets outside of any context, but that would compromise the security of the associated Slack channel for:
  • Send messages as @<!---->CircleCI notifications
  • Send messages to channels @<!---->CircleCI notifications isn't a member of
  • Upload, edit, and delete files as CircleCI notifications
• Using CircleCI logic statements to conditionally run jobs when CIRCLECI_USERNAME or CIRCLE_PR_USERNAME env vars are in a list of allowed users. However, that would be something difficult to maintain, and there's no other way to check the user's role.

Given that we don't find those trade-offs to be acceptable, we remove the integration for now.

Closes #4902

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[waiting-for-dev]

With the second commit fixing the libvips installation, we'll close #33.