chore: Removed outdated Slack reference (#253)

CONTRIBUTING.md

@@ -35,13 +35,6 @@ 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).
-
 ## PR Guidelines
 
 ### Version Support