Remove component style override

- style added 20px margin top to the intervention component
- component is already wrapped in an element that has 20px margin top, so this rule did nothing
- applications shouldn't style components, changes should be made directly to components or component options

app/assets/stylesheets/components/_banner.scss

@@ -40,7 +40,3 @@
   max-width: 30em;
   padding-top: govuk-spacing(2);
 }
-
-.gem-c-intervention {
-  margin-top: govuk-spacing(4);
-}