Delete "/system/cache/debug_@*.txt" files from cron

Imported From: http://groups.google.com/group/in-portal-dev/browse_thread/thread/812fa3b0a2fe1126#

During development (while in debug mode) we are creating debug_@SID@.txt files, which contain a lot of information. Sometimes we don't delete them automatically when session is expired. I propose we create cron event, that will delete all debugger report files of expired sessions.