Remove WebRequest::checkUrlExtension

It has been deprecated since 1.35 and it is unused.

Change-Id: I3afc838fbcb09a389922f6e8bec2cd2752fa5460

RELEASE-NOTES-1.39

@@ -277,6 +277,7 @@ because of Phabricator reports.
   internal in 1.35.
 * SpecialMute::isTargetBlacklisted(), deprecated in 1.35, has been removed. Use
   ::isTargetMuted() instead.
+* WebRequest::checkUrlExtension(), deprecated in 1.35, has been removed.
 * …
 
 === Deprecations in 1.39 ===

includes/WebRequest.php

@@ -1175,20 +1175,6 @@ class WebRequest {
 		$this->getSession()->set( $key, $data );
 	}
 
-	/**
-	 * This function formerly did a security check to prevent an XSS
-	 * vulnerability in IE6, as documented in T30235. Since IE6 support has
-	 * been dropped, this function now returns true unconditionally.
-	 *
-	 * @deprecated since 1.35
-	 * @param array $extList
-	 * @return bool
-	 */
-	public function checkUrlExtension( $extList = [] ) {
-		wfDeprecated( __METHOD__, '1.35' );
-		return true;
-	}
-
 	/**
 	 * Parse the Accept-Language header sent by the client into an array
 	 *