Remove Beta label from Statistics finders

These were in beta as we migrated Office for National Statistics to use
GOV.UK for all their statistics publications an announcements.

This transition has finished and ONS are happily publishing exclusively on
GOV.UK, so we don't consider it beta anymore.

https://www.agileplannerapp.com/boards/105200/cards/6138
Merge pull request #1772 from alphagov/remove-statistics-beta-label

Remove Beta label from Statistics finders