Remove shadowing of Db class instance variables

The instance variables $_db, $_table, and $_columnMap of the Zend_Log_Writer_Db class were redeclared in the FireGento_Logger_Model_Db class with a different visibility. This resulted in the following PHP errors:

    PHP Fatal error:  Access level to FireGento_Logger_Model_Db::$_db must be protected (as in class Zend_Log_Writer_Db) or weaker in /MAGENTO_ROOT/app/code/community/FireGento/Logger/Model/Db.php on line 29
    PHP Fatal error:  Access level to FireGento_Logger_Model_Db::$_table must be protected (as in class Zend_Log_Writer_Db) or weaker in /MAGENTO_ROOT/app/code/community/FireGento/Logger/Model/Db.php on line 29
    PHP Fatal error:  Access level to FireGento_Logger_Model_Db::$_columnMap must be protected (as in class Zend_Log_Writer_Db) or weaker in /MAGENTO_ROOT/app/code/community/FireGento/Logger/Model/Db.php on line 29

These errors can be resolved by changing the visibility of the instance variables from private to public.

However, there is no need to redeclare the instance variables, so I have removed them.