Remove external link tracking

This isn't used anywhere and isn't an approach we should use.
Remove hash navigation fix

This isn't used anywhere. It also has logic in it to ensure the sticky
heading bar doesn't cover the content. The header bar isn't sticky and
hasn't been for ages.
Remove popup javascript and styles

Fairly sure these aren't used any more.
Remove unused button hover and focus JavaScript

This JavaScript was adding classes that were never used anywhere. The
only place that they were used, the header, no longer has an input with
the `.button` class. Removed the CSS from the header which wouldn't have
targeted anything.
Merge pull request #378 from alphagov/remove-js

Remove unused code