Remove outdated docs for `shimLinksWithButtonRole`
Remove outdated docs for `shimLinksWithButtonRole`

Customisation of `shimLinksWithButtonRole` was removed in v5.0.1
(see commit e882086f8e1ed00ee86c7b276b477b498ca6035a).

The docs have been updated to no longer imply that it can still
be customised.

alphagov/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit