Replace Drupal Set Message Filter Cache Cleared with Watchdog message

The glossary module in _glossary_clear_cache() function the system puts out the drupal_set_message filter cache being cleared.

This is confusing to users who are updating the taxonomies and maybe are not administrators and if there are more than one term in the vocabulary then the message will appear 'X' times with X being the number of terms in the vocabulary.

The patch proposed below makes this message a watchdog message so that if the administrator really wishes to see this they can check the log reports.