Remove APCu PHP functions link from performance.rst

Removed link to APCu PHP functions documentation.

Author: nacholibre

performance.rst

@@ -387,7 +387,6 @@ Learn more
 .. _`byte code caches`: https://en.wikipedia.org/wiki/List_of_PHP_accelerators
 .. _`OPcache`: https://www.php.net/manual/en/book.opcache.php
 .. _`Composer's autoloader optimization`: https://getcomposer.org/doc/articles/autoloader-optimization.md
-.. _`APCu PHP functions`: https://www.php.net/manual/en/ref.apcu.php
 .. _`cachetool`: https://github.com/gordalina/cachetool
 .. _`open_basedir`: https://www.php.net/manual/ini.core.php#ini.open-basedir
 .. _`Blackfire`: https://blackfire.io/docs/introduction?utm_source=symfony&utm_medium=symfonycom_docs&utm_campaign=performance