Token needed for slack notifications has been removed

[NicholasEllul]

Description

Until development continues on the MetaMask Desktop project, I am removing the slack secrets that exist in CircleCI. These secrets are under the following format:

By creating this issue, I am documenting that it should be addressed if the project ever gets picked up again.

Note

Please note that we also have access to the following unauthenticated endpoint for sending slack messages: https://github.com/MetaMask/action-npm-publish/blob/11e1f5e15b5a6721ee0c5599d9cd9731305e83c4/action.yml#L57-L71

This endpoint can be accessed using the following org secret:
https://github.com/MetaMask/snaps/blob/5cd6e723abc102cdccfa9b961ecce2ec9b5b848b/.github/workflows/publish-release.yml#L100