Remove big_number import from dashboard

It was added in:

https://github.com/alphagov/notifications-admin/pull/734

but neither macro is called so assuming it can be
removed.

app/templates/views/dashboard/dashboard.html

@@ -1,6 +1,5 @@
 {% extends "withnav_template.html" %}
 
-{% from "components/big-number.html" import big_number, big_number_with_status %}
 {% from "components/show-more.html" import show_more %}
 {% from "components/message-count-label.html" import message_count_label %}
 {% from "components/table.html" import list_table, field, right_aligned_field_heading, hidden_field_heading %}