Fix PHP Recoverable Error in cron job. Move database code to resource mo...

...del and do not load logs into memory before deleting them.

I started getting this error after updating:

> Recoverable Error: Argument 1 passed to Firegento_Logger_Model_Observer::cleanLogs() must be an instance of Varien_Event_Observer, instance of Mage_Cron_Model_Schedule given  in /var/www/..../.modman/Logger/app/code/community/Firegento/Logger/Model/Observer.php on line 22

Also the method of deleting could quite easily run out of memory if the logs got flooded somehow and was generally very inefficient. Loaded all logs into collection with full-blow models and delete individually while not actually reading any of the data..