Make only the scrollable table focusable

The JS clones the scrollable table so was passing
its attributes across to the fixed one (which
provides the row headings).

This bug was pushed in:

https://github.com/alphagov/notifications-admin/pull/3637

app/assets/javascripts/fullscreenTable.js

@@ -48,6 +48,7 @@
             .clone()
             .addClass('fullscreen-fixed-table')
             .removeClass('fullscreen-scrollable-table')
+            .removeAttr('role aria-labelledby tabindex')
             .attr('aria-hidden', true)
         )
         .append(