Remove references to Spotify FOSS Slack

The FOSS Slack is being shut down. We will continue use GitHub issues, discussions, etc. for receiving feedback and bug reports.

Author: BalestraPatrick

README.md

@@ -7,7 +7,6 @@ _XCRemoteCache is a remote cache tool for Xcode projects. It reuses target artif
 
 [![Build Status](https://github.com/spotify/XCRemoteCache/workflows/CI/badge.svg)](https://github.com/spotify/XCRemoteCache/workflows/CI/badge.svg)
 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](LICENSE)
-[![Slack](https://slackin.spotify.com/badge.svg)](https://slackin.spotify.com)
 [![Docs](https://github.com/spotify/XCRemoteCache/workflows/Docs/badge.svg)](https://spotify.github.io/XCRemoteCache/documentation/xcremotecache/)
 
 - [How and Why?](#how-and-why)
@@ -499,8 +498,6 @@ The zip package will be generated at `releases/XCRemoteCache.zip`.
 
 Create a [new issue](https://github.com/spotify/XCRemoteCache/issues/new) with as many details as possible.
 
-Reach us at the `#xcremotecache` channel in [Slack](https://slackin.spotify.com/).
-
 ## Contributing
 
 We feel that a welcoming community is important and we ask that you follow Spotify's