Remove VectorLanguage A/B test code (WgVectorLanguageInHeaderTreatmentABTest)

The removal here is actually relatively straightforward.
References inside OverridableConfigRequirementTest are kept as
the A/B test capability remains after this removal and the tests
here do not relate to the code removed, just examples of how it
can be used.

Bug: T302027
Change-Id: I2dd92adaff1221d8213723e9ee60aa02787d4cc7

includes/Constants.php

@@ -98,17 +98,6 @@ final class Constants {
 	 */
 	public const REQUIREMENT_LANGUAGE_IN_HEADER = 'LanguageInHeader';
 
-	/**
-	 * Defines whether or not the Language in header A/B test is running. See
-	 * https://phabricator.wikimedia.org/T280825 for additional detail about the test.
-	 *
-	 * Note well that if the associated config value is falsy, then we fall back to choosing the
-	 * language treatment based on the `VectorLanguageInHeader` config variable.
-	 *
-	 * @var string
-	 */
-	public const CONFIG_LANGUAGE_IN_HEADER_TREATMENT_AB_TEST = 'VectorLanguageInHeaderTreatmentABTest';
-
 	/**
 	 * @var string
 	 */

includes/ServiceWiring.php

@@ -53,8 +53,7 @@ return [
 				$services->getCentralIdLookupFactory()->getNonLocalLookup(),
 				Constants::CONFIG_KEY_LANGUAGE_IN_HEADER,
 				Constants::REQUIREMENT_LANGUAGE_IN_HEADER,
-				null,
+				null
-				Constants::CONFIG_LANGUAGE_IN_HEADER_TREATMENT_AB_TEST
 			)
 		);
 
@@ -63,13 +62,9 @@ return [
 		// Temporary T286932 - remove after languages A/B test is finished.
 		$requirementName = 'T286932';
 
-		// MultiConfig checks each config in turn, allowing us to override the main config for specific keys. In this
+		// MultiConfig checks each config in turn, allowing us to override the main config for specific keys.
-		// case, override the "VectorLanguageInHeaderABTest" configuration value so that the following requirement
-		// always buckets the user as if the language treatment A/B test were running.
 		$config = new MultiConfig( [
-			new HashConfig( [
+			new HashConfig( [] ),
-				Constants::CONFIG_LANGUAGE_IN_HEADER_TREATMENT_AB_TEST => true,
-			] ),
 			$services->getMainConfig(),
 		] );
 
@@ -81,8 +76,7 @@ return [
 				$services->getCentralIdLookupFactory()->getNonLocalLookup(),
 				Constants::CONFIG_KEY_LANGUAGE_IN_HEADER,
 				$requirementName,
-				/* $overrideName = */ '',
+				/* $overrideName = */ ''
-				Constants::CONFIG_LANGUAGE_IN_HEADER_TREATMENT_AB_TEST
 			)
 		);
 

skin.json

@@ -539,10 +539,6 @@
 			},
 			"description": "@var When `VectorLanguageAlertInSidebar` is enabled, the language switch alert box is shown in the sidebar."
 		},
-		"VectorLanguageInHeaderTreatmentABTest": {
-			"value": false,
-			"description": "@var boolean Enables or disables the language in header treatment A/B test. See https://phabricator.wikimedia.org/T280825 and associated tasks for additional detail."
-		},
 		"VectorStickyHeader": {
 			"value": {
 				"logged_in": true,

tests/phpunit/unit/FeatureManagement/Requirements/OverridableConfigRequirementTest.php

@@ -322,7 +322,7 @@ class OverridableConfigRequirementTest extends \MediaWikiUnitTestCase {
 	) {
 		$config = new HashConfig( [
 			Constants::CONFIG_KEY_LANGUAGE_IN_HEADER => $configValue,
-			Constants::CONFIG_LANGUAGE_IN_HEADER_TREATMENT_AB_TEST => $abValue,
+			$testName => $abValue,
 		] );
 
 		$user = $this->createMock( User::class );