Update data format for platform admin report

The format of dates returned from API for the "Monthly notification
statuses for live services" report has changed, so we no longer need to
format the dates here.

https://github.com/alphagov/notifications-api/pull/3279

app/main/views/platform_admin.py

@@ -260,9 +260,6 @@ def notifications_sent_by_service():
         ]
         result = notification_api_client.get_notification_status_by_service(start_date, end_date)
 
-        for row in result:
-            row[0] = datetime.strptime(row[0], '%a, %d %b %Y %X %Z').strftime('%Y-%m-%d')
-
         return Spreadsheet.from_rows([headers] + result).as_csv_data, 200, {
             'Content-Type': 'text/csv; charset=utf-8',
             'Content-Disposition': 'attachment; filename="{} to {} notification status per service report.csv"'.format(