chore: Removed outdated Slack reference (#233)

Author: jsumners-nr

Diff for: CONTRIBUTING.md

@@ -24,7 +24,3 @@ Keep in mind that when you submit your Pull Request, you'll need to sign the CLA
 
 For more information about CLAs, please check out Alex Russell’s excellent post,
 [“Why Do I Need to Sign This?”](https://infrequently.org/2008/06/why-do-i-need-to-sign-this/).
-
-## Slack
-
-We host a public Slack with a dedicated channel for contributors and maintainers of open source projects hosted by New Relic.  If you are contributing to this project, you're welcome to request access to the #oss-contributors channel in the newrelicusers.slack.com workspace. To request access, please use this [link](https://join.slack.com/t/newrelicusers/shared_invite/zt-1ayj69rzm-~go~Eo1whIQGYnu3qi15ng).