Remove CHANGELOG entry for change that was backported (opensearch-project#16514)

This was backported in opensearch-project#15703 so it should no longer be listed in the
changelog.

Signed-off-by: Andrew Ross <[email protected]>

Author: andrross

Diff for: CHANGELOG.md

@@ -5,7 +5,6 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 
 ## [Unreleased 2.x]
 ### Added
-- MultiTermQueries in keyword fields now default to `indexed` approach and gated behind cluster setting ([#15637](https://github.com/opensearch-project/OpenSearch/pull/15637))
 - Latency and Memory allocation improvements to Multi Term Aggregation queries ([#14993](https://github.com/opensearch-project/OpenSearch/pull/14993))
 - Add support for restoring from snapshot with search replicas ([#16111](https://github.com/opensearch-project/OpenSearch/pull/16111))
 - Add logic in master service to optimize performance and retain detailed logging for critical cluster operations. ([#14795](https://github.com/opensearch-project/OpenSearch/pull/14795))