doc: Remove outdated Slack Cert mention from tools (!17717) · Merge requests · GitLab.com / www-gitlab-com · GitLab

Lucas Charles requested to merge theoretick-master-patch-92213 into master Jan 02, 2019

Removes outdated reference to now closed beta for "Slack Certification".

Per https://get.slack.help/hc/en-us/articles/115002636666-Slack-Certification-beta-

The Slack Certification beta is now closed.