Remove deprecated $govuk-border-width-form-element-error variable

[36degrees]

What

Remove the $govuk-border-width-form-element-error setting, and update any references in the code to use $govuk-border-width-form-element, or to just set the border-color as appropriate.

Why

We deprecated $govuk-border-width-form-element-error in #1870 the border width of form elements in their error state ($govuk-border-width-form-element-error) is now the same as form elements in their normal state ($govuk-border-width-form-element) – both are 2px.

This isn't something we think users need to be able to configure, and we don't really want the complexity of trying to support different border widths for the error state (for example, the box shadow override that we removed in 06623f8 would need to be conditionally added if the user changed $govuk-border-width-form-element in order to maintain a consistent focus state).

Who needs to know about this

Developers, Tech Writer

Done when

• $govuk-border-width-form-element-error has been removed

[EoinShaughnessy]

@vanitabarrett @36degrees Do we need to update the title and description to say 'variable' instead of 'setting'? (Asking in case any users ever end up on this PR.)