revert performance improvements in userExists() because it breaks new user creation

lib/user.php

@@ -345,17 +345,13 @@ class OC_User {
 	 * @return boolean
 	 */
 	public static function userExists($uid){
-		static $user_exists_checked = null;
-		if (!is_null($user_exists_checked)) {
-			return $user_exists_checked;
-		}
 		foreach(self::$_usedBackends as $backend){
 			$result=$backend->userExists($uid);
 			if($result===true){
-				return $user_exists_checked = true;
+				return true;
 			}
 		}
-		return $user_exists_checked = false;
+		return false;
 	}
 
 	/**