Do not set WP_CACHE_KEY_SALT

It shouldn't be necessary in our setup, and it just makes keys
needlessly longer.

docker/wp-config.php

@@ -126,7 +126,6 @@ define('AUTH_SALT',         $noblogs_config['secrets']['auth_salt']);
 define('SECURE_AUTH_SALT',  $noblogs_config['secrets']['secure_auth_salt']);
 define('LOGGED_IN_SALT',    $noblogs_config['secrets']['logged_in_salt']);
 define('NONCE_SALT',        $noblogs_config['secrets']['nonce_salt']);
-define('WP_CACHE_KEY_SALT', $noblogs_config['secrets']['wp_cache_key_salt']);
 
 /**#@-*/