Update CHANGELOG

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CHANGELOG

@@ -14,7 +14,6 @@ v 8.4.0 (unreleased)
   - Revert back upvote and downvote button to the issue and MR pages
   - Swap position of Assignee and Author selector on Issuables (Zeger-Jan van de Weg)
   - Fix version check image in Safari
-  - Enable "Add key" button when user fills in a proper key (Stan Hu)
   - Show 'All' tab by default in the builds page
 v 8.3.3 (unreleased)