add changeset, remove changelog

Author: Brian Chen

packages/firestore/CHANGELOG.md

@@ -1,8 +1,6 @@
 # Unreleased
 - [fixed] Fixed an issue that may have prevented the client from connecting 
   to the backend immediately after a user signed in.
-- [fixed] Added support for using `set()` with merge options when using
-  `FirestoreDataConverter`.
 
 # Released
 - [changed] All known failure cases for Indexed-related crashes have now been