Remove the letter validation preview endpoints

We can use the ‘Uploads’ feature to check if letters are printable now.

This code works in a completely different way, so if we kept it we’d
have to maintain two different code paths, and make sure that they
didn’t diverge.

Also deletes the related HTML templates.

app/templates/views/platform-admin/_base_template.html

@@ -24,7 +24,6 @@
           ('Find users by email', url_for('main.find_users_by_email')),
           ('Email Complaints', url_for('main.platform_admin_list_complaints')),
           ('Returned letters', url_for('main.platform_admin_returned_letters')),
-          ('Letter validation preview', url_for('main.platform_admin_letter_validation_preview')),
           ('Clear cache', url_for('main.clear_cache')),
         ] %}
           <li>