Move CRM_Core_Session::setStatus calls out of CRM_Core_Tag_BAO layer

When the Tag_BAO layer calls setStatus it removes the developer's control over status messages. Instead, the burden should always be placed on the calling context to set the appropriate status, if necessary.

I ran into this when our AJAX call that used the civicrm_api to delete a tag was generating status messages on the next page (because the api in turn used the BAO layer). By the time the user loads another page, these message are no longer relevant and can become confusing.