Remove tsdb_doc_values_optimized_merge feature flag.

[martijnvg]

All the development work has been completed in #126111 and no regressions have been observer in nightly benchmarks.

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)

[martijnvg]

@elasticmachine update branch

[martijnvg]

Unrelated test failure:

�_bk;t=1748246171941�REPRODUCE WITH: ./gradlew ":qa:x-pack-rest-test:yamlRestTest" --tests "co.elastic.elasticsearch.qa.rest.ServerlessXpackRestIT.test {yaml=ml/set_upgrade_mode/Setting upgrade_mode to enabled}" -Dtests.seed=20E5155D86BD2FE9 -Dtests.locale=ff-Adlm-NG -Dtests.timezone=EST5EDT -Druntime.java=24

�_bk;t=1748246172041�

�_bk;t=1748246172045�ServerlessXpackRestIT > test {yaml=ml/set_upgrade_mode/Setting upgrade_mode to enabled} FAILED

�_bk;t=1748246172045�    java.lang.AssertionError: 2 active tasks found:

�_bk;t=1748246172045�    cluster:admin/xpack/ml/job/reset opvpIxRGT3SVzozFUuLpAg:43468  -                             transport  1748246138451 07:55:38 33s         127.0.0.1 yamlRestTest-2-kNOXrja job-set-upgrade-mode-job

�_bk;t=1748246172045�    cluster:admin/xpack/ml/job/reset wOzmi_NaQtmXA5K8d4jsPw:118290 opvpIxRGT3SVzozFUuLpAg:43468  transport  1748246138452 07:55:38 33s         127.0.0.1 index-XUOLMXd          job-set-upgrade-mode-job

�_bk;t=1748246172045�     expected:<0> but was:<2>

�_bk;t=1748246172045�        at __randomizedtesting.SeedInfo.seed([20E5155D86BD2FE9:A8B12A8728414211]:0)

�_bk;t=1748246172045�        at org.junit.Assert.fail(Assert.java:89)

�_bk;t=1748246172045�        at org.junit.Assert.failNotEquals(Assert.java:835)

�_bk;t=1748246172045�        at org.junit.Assert.assertEquals(Assert.java:647)

�_bk;t=1748246172045�        at org.elasticsearch.test.rest.ESRestTestCase.lambda$waitForPendingTasks$7(ESRestTestCase.java:717)

�_bk;t=1748246172045�        at org.elasticsearch.test.ESTestCase.assertBusy(ESTestCase.java:1481)

�_bk;t=1748246172045�        at org.elasticsearch.test.rest.ESRestTestCase.waitForPendingTasks(ESRestTestCase.java:686)

�_bk;t=1748246172045�        at org.elasticsearch.xpack.test.rest.AbstractXPackRestTest.cleanup(AbstractXPackRestTest.java:122)

�_bk;t=1748246172045�        at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104)

�_bk;t=1748246172045�        at java.base/java.lang.reflect.Method.invoke(Method.java:565)

�_bk;t=1748246172045�        at com.carrotsearch.randomizedtesting.RandomizedRunner.invoke(RandomizedRunner.java:1763)

�_bk;t=1748246172045�        at com.carrotsearch.randomizedtesting.RandomizedRunner$10.evaluate(RandomizedRunner.java:1004)

�_bk;t=1748246172045�        at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)

�_bk;t=1748246172045�        at org.junit.rules.RunRules.evaluate(RunRules.java:20)

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	8.19