Remove outdated default index mapping error message

This error message is no longer relevant and should not be used now.

Closes: #7007

Signed-off-by: Lukáš Vlček <lukas.vlcek@aiven.io>

CHANGELOG.md

@@ -65,6 +65,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Remove LegacyESVersion.V_7_10_ Constants ([#5018](https://github.com/opensearch-project/OpenSearch/pull/5018))
 - Remove Version.V_1_ Constants ([#5021](https://github.com/opensearch-project/OpenSearch/pull/5021))
 - Remove custom Map, List and Set collection classes ([#6871](https://github.com/opensearch-project/OpenSearch/pull/6871))
+- Remove outdated DEFAULT_MAPPING_ERROR_MESSAGE error message ([#7008](https://github.com/opensearch-project/OpenSearch/pull/7008))
 
 ### Fixed
 - Fix 'org.apache.hc.core5.http.ParseException: Invalid protocol version' under JDK 16+ ([#4827](https://github.com/opensearch-project/OpenSearch/pull/4827))
@@ -128,4 +129,4 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Security
 
 [Unreleased 3.0]: https://github.com/opensearch-project/OpenSearch/compare/2.x...HEAD
-[Unreleased 2.x]: https://github.com/opensearch-project/OpenSearch/compare/2.5...2.x
+[Unreleased 2.x]: https://github.com/opensearch-project/OpenSearch/compare/2.5...2.x

server/src/main/java/org/opensearch/index/mapper/MapperService.java

@@ -172,9 +172,6 @@ public enum MergeReason {
     );
 
     private static final DeprecationLogger deprecationLogger = DeprecationLogger.getLogger(MapperService.class);
-    static final String DEFAULT_MAPPING_ERROR_MESSAGE = "[_default_] mappings are not allowed on new indices and should no "
-        + "longer be used. See [https://www.elastic.co/guide/en/elasticsearch/reference/current/breaking-changes-7.0.html"
-        + "#default-mapping-not-allowed] for more information.";
 
     private final IndexAnalyzers indexAnalyzers;