Remove the feedback survey link from the email template

## Problem

The [`instructor_email_digest` template in Mandrill](https://mandrillapp.com/templates/code?id=instructor-email-digest) currently contains **two** links to our beta feedback survey: . Both the **Take our survey** link and the **Share Feedback** button:


These links are for beta users. They should be removed before we roll the feature out to everyone (https://github.com/hypothesis/lms/issues/5367).

## Solution

The template just needs to be edited directly in Mandrill. Log in to Mandrill and follow the link above or navigate to **Outbound > Templates > instructor_email_digest**. Use [the admin page](https://lms.hypothes.is/admin/email) to test the modified template.