Issue #2067891 by dawehner: Remove override of check_plain().

core/tests/Drupal/Tests/Core/Asset/CssCollectionRendererUnitTest.php

@@ -33,14 +33,6 @@ function variable_get($variable, $default) {
 
 }
 
-if (!function_exists('check_plain')) {
-
-  function check_plain($text) {
-    return $text;
-  }
-
-}
-
 }