Remove outdated default index mapping error message (#7008)

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

Closes: #7007

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

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;