Remove unused textbox imports

When we converted textboxes on Notify to govuk-frontend design system
style with @tombye , we did not remove the imports.
This work is now done in this commit.

Some things that will still stay in our repo for now:

textbox macro in our components folder and associated css
textboxes which are really textarea fields (🤷 how do they work)
textboxes on the styleguide page

We should revisit this when we convert textarea fields
to govuk-frontend.

app/templates/views/api/callbacks.html

@@ -1,5 +1,4 @@
 {% extends "withnav_template.html" %}
-{% from "components/textbox.html" import textbox %}
 {% from "components/page-header.html" import page_header %}
 {% from "components/page-footer.html" import page_footer %}
 {% from "components/table.html" import mapping_table, row, text_field, edit_field, optional_text_field with context %}