Remove the code to handle optional address placeholders

Optional address placeholders aren’t a thing for one-off letters any
more, so we can tidy up the code a bit by removing the parts of the flow
that are accounting for them.

app/templates/views/send-test.html

@@ -25,7 +25,6 @@
       <div class="govuk-grid-column-full {% if form.placeholder_value.label.text == 'phone number' %}extra-tracking{% endif %}">
         {{ textbox(
           form.placeholder_value,
-          hint='Optional' if optional_placeholder else None,
           width='1-1',
         ) }}
       </div>