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/platform-admin/services.html

@@ -1,5 +1,4 @@
 {% extends "views/platform-admin/_base_template.html" %}
-{% from "components/textbox.html" import textbox %}
 {% from "components/page-footer.html" import page_footer %}
 {% from "components/big-number.html" import big_number, big_number_with_status %}
 {% from "components/message-count-label.html" import message_count_label %}