Update no-underline modifier/mixin to work in IE11

IE11 does not support the CSS Text Decoration Module Level 3 ‘text-decoration styling’ properties [1] [2], and only recognises `text-decoration` as a regular property with values of none, underline, etc – as defined in CSS2 [3].

As such, using `text-decoration-line` to ‘unset’ the underline does not work in IE11. We want to avoid re-setting the `text-decoration` property because when treated as shorthand (in browsers that support `text-decoration-styling` this resets the `text-decoration-thickness` property to `auto`.

Instead, use `:not(:hover):not(:active)` to remove the text-decoration except when hovered/active. `:not` is supported in IE9 and above. This means that the `no-underline` mixin / modifier will have no effect in IE8.

Unfortunately the pseudo-class plugin we use to display links in different states does not correctly handle `:not` when used like this, so the examples in the links page do not render correctly. Revert the changes to this example, adding a simpler example instead.

[1]: https://www.w3.org/TR/css-text-decor-3/#text-decoration
[2]: https://caniuse.com/text-decoration
[3]: https://www.w3.org/TR/2009/CR-CSS2-20090908/text.html#propdef-text-decoration

# Conflicts:
#	app/views/examples/links/index.njk

src/govuk/helpers/_links.scss

@@ -288,16 +288,7 @@
 /// @access public
 
 @mixin govuk-link-style-no-underline {
-  &:link {
-    text-decoration-line: none;
-  }
-
-  &:hover,
-  &:active {
-    text-decoration-line: underline;
-  }
-
-  &:focus {
+  &:not(:hover):not(:active) {
     text-decoration: none;
   }
 }