Remove unreleased note from changelog

[ci skip]

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
-v 8.4.0 (unreleased)
+v 8.4.0
   - Allow LDAP users to change their email if it was not set by the LDAP server
   - Ensure Gravatar host looks like an actual host
   - Consider re-assign as a mention from a notification point of view