Remove settings and mixins for building IE8 specific stylesheets

[romaricpascal]

What

Remove the Sass settings and mixins used for building IE8 specific stylesheets, including:

• the $govuk-is-ie8 setting
• the $govuk-ie8-breakpoint setting
• the govuk-if-ie8 mixin
• the govuk-not-ie8 mixin

Why

We're dropping support for IE8 in v5 so we no longer need the mixins for outputing IE8 specific code

Who needs to work on it

Developers

Who needs to review it

Developers

Done when

• The mixins and settings have been removed from the code
• References to these mixins and settting in the docs have been removed

[querkmachine]

We've decided to handle the docs changes after all of the IE removals have taken place, rather than doing them piecemeal, so I'm closing this as done.