Remove duplicate button styles
Remove duplicate button styles

- Ensure buttons are full-width for smaller screens
- These styles exist in _buttons.scss

alphagov/govuk_elements,joelanman/govuk_elements,TFOH/govuk_elements,alphagov/govuk_elements,gavboulton/govuk_elements,alphagov/govuk_elements,alphagov/govuk_elements,joelanman/govuk_elements,gavboulton/govuk_elements,gavboulton/govuk_elements,gavboulton/govuk_elements,TFOH/govuk_elements