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/main/views/platform_admin.py

@@ -386,77 +386,6 @@ def platform_admin_returned_letters():
     )
 
 
-@main.route("/platform-admin/letter-validation-preview", methods=["GET", "POST"])
-@user_is_platform_admin
-def platform_admin_letter_validation_preview():
-    return letter_validation_preview(from_platform_admin=True)
-
-
-@main.route("/services/<uuid:service_id>/letter-validation-preview", methods=["GET", "POST"])
-@user_has_permissions()
-def service_letter_validation_preview(service_id):
-    return letter_validation_preview(from_platform_admin=False)
-
-
-def letter_validation_preview(from_platform_admin):
-    message, pages, passed_validation, error_code = None, [], None, None
-    form = PDFUploadForm()
-
-    view_location = 'views/platform-admin/letter-validation-preview.html' \
-        if from_platform_admin else'views/letter-validation-preview.html'
-
-    if form.validate_on_submit():
-        pdf_file = form.file.data
-        virus_free = antivirus_client.scan(pdf_file)
-
-        if not virus_free:
-            return render_template(
-                view_location,
-                form=form, message="Document did not pass the virus scan",
-                pages=pages, passed_validation=passed_validation
-            ), 400
-
-        try:
-            if len(pdf_file.read()) > (2 * 1024 * 1024):
-                return render_template(
-                    view_location,
-                    form=form,
-                    message="File must be less than 2MB",
-                    pages=pages, passed_validation=passed_validation
-                ), 400
-            pdf_file.seek(0)
-            response = validate_letter(pdf_file)
-            response.raise_for_status()
-            if response.status_code == 200:
-                pages, message = response.json()["pages"], response.json()["message"],
-                passed_validation = response.json()["result"]
-                invalid_pages = response.json().get('invalid_pages')
-                page_count = len(pages)
-                if page_count > LETTER_MAX_PAGE_COUNT:
-                    message = "letter-too-long"
-                    passed_validation = False
-
-                if not passed_validation:
-                    error_code = message
-                    message = get_letter_validation_error(
-                        message, invalid_pages=invalid_pages, page_count=page_count
-                    )
-        except RequestException as error:
-            if error.response and error.response.status_code == 400:
-                message = "Something was wrong with the file you tried to upload. Please upload a valid PDF file."
-                return render_template(
-                    view_location,
-                    form=form, message=message, pages=pages, passed_validation=passed_validation
-                ), 400
-            else:
-                raise error
-
-    return render_template(
-        view_location,
-        form=form, message=message, pages=pages, passed_validation=passed_validation, error_code=error_code
-    )
-
-
 @main.route("/platform-admin/clear-cache", methods=['GET', 'POST'])
 @user_is_platform_admin
 def clear_cache():