Hi @s.u,
You can remove the /opt/kaltura/app/start dir if you wish. That will not cause the platform to malfunction in any way but if you do that, I’d also remove the rewrite rule in /opt/kaltura/app/configurations/apache/conf.d/enabled.kaltura.conf:
RewriteRule ^/$ /start/index.php [R]
As for /admin_console, see /opt/kaltura/app/configurations/apache/conf.d/admin.conf. You may limit access to it using standard Apache mechanisms [modules], see:
https://httpd.apache.org/docs/2.4/howto/access.html