Issue #2297487 by er.pushpinderrana, marcingy: Remove the check_plain function.

core/includes/bootstrap.inc

@@ -715,19 +715,6 @@ function format_string($string, array $args = array()) {
   return String::format($string, $args);
 }
 
-/**
- * Encodes special characters in a plain-text string for display as HTML.
- *
- * @see drupal_validate_utf8()
- * @ingroup sanitization
- *
- * @deprecated in Drupal 8.x-dev, will be removed before Drupal 8.0.
- *   Use \Drupal\Component\Utility\String::checkPlain().
- */
-function check_plain($text) {
-  return String::checkPlain($text);
-}
-
 /**
  * Checks whether a string is valid UTF-8.
  *