Remove unused data-transaction-slug attribute
Remove unused data-transaction-slug attribute

Was added originally in

- https://github.com/alphagov/frontend/pull/419
- https://github.com/alphagov/static/pull/310

And removed in https://github.com/alphagov/static/pull/323

But I think we forgot to remove this attribute from frontend.

Doing this means we wont have to add data attributes to our button component :)

alphagov/frontend,alphagov/frontend,alphagov/frontend,alphagov/frontend