Remove display block style on hint component

In v3.8.0 we added the ability for block level elements to be
rendered inside hints by switching from using a `span` to a `div`.

However, we kept the `display: block` styling (which would now be
redundant as this is the default for divs). This meant we weren't
forcing users to update their HTML and we could avoid a breaking change.

We now want to tidy up that redundant CSS as part of v4.0.0.

src/govuk/components/hint/_index.scss

@@ -2,8 +2,6 @@
  .govuk-hint {
  @include govuk-font($size: 19);
 
- display: block;
-
  margin-bottom: govuk-spacing(3);
 
  color: $govuk-secondary-text-colour;