[8.19] Remove tsdb_doc_values_optimized_merge feature flag. (elastic#128435)

This PR should have merged end of May. But PR CI got stuck and never got merged.
This caused the optimized doc value merging to not be enabled in 8.19. So this PR turned into a bug fix instead of a backport.

Backports the following commits to 8.19:

Remove tsdb_doc_values_optimized_merge feature flag. (Remove tsdb_doc_values_optimized_merge feature flag. elastic#128435)

Author: martijnvg

docs/changelog/128459.yaml

@@ -0,0 +1,5 @@
+pr: 128459
+summary: "[8.19] Remove forgotten `tsdb_doc_values_optimized_merge` feature flag that caused an optimization to be disabled that should have been enabled"
+area: Codec
+type: bug
+issues: []

server/src/main/java/org/elasticsearch/index/codec/tsdb/es819/ES819TSDBDocValuesFormat.java

@@ -13,7 +13,6 @@
 import org.apache.lucene.codecs.DocValuesProducer;
 import org.apache.lucene.index.SegmentReadState;
 import org.apache.lucene.index.SegmentWriteState;
-import org.elasticsearch.common.util.FeatureFlag;
 
 import java.io.IOException;
 
@@ -59,14 +58,10 @@ public class ES819TSDBDocValuesFormat extends org.apache.lucene.codecs.DocValues
 
     // Default for escape hatch:
     static final boolean OPTIMIZED_MERGE_ENABLE_DEFAULT;
-    static final FeatureFlag TSDB_DOC_VALUES_OPTIMIZED_MERGE = new FeatureFlag("tsdb_doc_values_optimized_merge");
     static final String OPTIMIZED_MERGE_ENABLED_NAME = ES819TSDBDocValuesConsumer.class.getName() + ".enableOptimizedMerge";
 
     static {
-        boolean optimizedMergeDefault = TSDB_DOC_VALUES_OPTIMIZED_MERGE.isEnabled();
-        OPTIMIZED_MERGE_ENABLE_DEFAULT = Boolean.parseBoolean(
-            System.getProperty(OPTIMIZED_MERGE_ENABLED_NAME, Boolean.toString(optimizedMergeDefault))
-        );
+        OPTIMIZED_MERGE_ENABLE_DEFAULT = Boolean.parseBoolean(System.getProperty(OPTIMIZED_MERGE_ENABLED_NAME, Boolean.TRUE.toString()));
     }
 
     private final boolean enableOptimizedMerge;