Delete all stored offline reports
        :return: List of reports that still require submission

def delete_offline_reports(self):
        """
        Delete all stored offline reports
        :return: List of reports that still require submission
        """
        reports = self.get_offline_reports()
        remaining_reports = reports[:]
        for report in reports:
            with open(report, 'r') as _f:
                try:
                    js = json.load(_f)
                except ValueError as e:
                    logging.error("%s. Deleting crash report.")
                    os.remove(report)
                    continue
                if js['SMTP Submission'] in ('Sent', 'Disabled') and js['HQ Submission'] in ('Sent', 'Disabled'):
                    # Only delete the reports which have been sent or who's upload method is disabled.
                    remaining_reports.remove(report)
                    try:
                        os.remove(report)
                    except OSError as e:
                        logging.error(e)

        self.logger.info('CrashReporter: Deleting offline reports. %d reports remaining.' % len(remaining_reports))
        return remaining_reports