Search toggle button in super navigation menu has a bottom border

[MartinJJones]

What

The search toggle button in the layout super navigation component header, has a blue bottom-border of 1px when in the default unopened state.

We should investigate the cause of this issue and come up with a suitable fix to remove the bottom-border.

Why

Does not appear to align with the intended design as shown in the component guide.

This is not visible in the component guide, to see this, you will need to visit a page on GOV.UK such as https://www.gov.uk/log-in-register-hmrc-online-services

[AshGDS]

For context, this was introduced when the header had a different design, so it's likely that updating this border was missed when the header design changed. The PR that added this border is here: #2413