Stop using mobile_top_margin in publications

- headings have this unusual behaviour where the mobile top margin is much larger
- this is slightly odd and inconsistent behaviour, so we're removing this option from the heading component and applying custom spacing in these pages instead using existing component options

app/views/content_items/publication.html.erb

@@ -52,7 +52,7 @@
         <section id="details">
           <%= render "govuk_publishing_components/components/heading", {
             text: t("publication.details"),
-            mobile_top_margin: true,
+            margin_bottom: 4
           } %>
 
           <%= render "govuk_publishing_components/components/govspeak", {