Drop VectorUseSimpleSearch config flag

For compatibility with gadgets always outputs the simpleSearch ID.

Bug: T254620
Change-Id: I0ed456f90f066129016f798a3ce65ba52bfc11e8

includes/VectorTemplate.php

@@ -511,7 +511,7 @@ class VectorTemplate extends BaseTemplate {
 		$config = $this->getConfig();
 		$props = [
 			'form-action' => $config->get( 'Script' ),
-			'form-id' => $config->get( 'VectorUseSimpleSearch' ) ? 'simpleSearch' : '',
+			'form-id' => 'simpleSearch',
 			'html-button-search-fallback' => $this->makeSearchButton(
 				'fulltext',
 				[ 'id' => 'mw-searchButton', 'class' => 'searchButton mw-fallbackSearchButton' ]

skin.json

@@ -131,9 +131,6 @@
 		}
 	},
 	"config": {
-		"VectorUseSimpleSearch": {
-			"value": true
-		},
 		"VectorUseIconWatch": {
 			"value": true
 		},