remove "letters stuck pending av" runbook

there's not anything we know we need to do now that we resolve stuck
letters automatically. Letters couuld still get into this state, so it's
worth alerting us. However, we don't have anything concrete that we know
how to fix these letters, so we should just remove the runbook entirely.

app/celery/scheduled_tasks.py

@@ -236,10 +236,9 @@ def check_if_letters_still_pending_virus_check():
         letter_ids = [(str(letter.id), letter.reference) for letter in letters]
 
         msg = f"""{len(letters)} precompiled letters have been pending-virus-check for over 90 minutes.
-            We couldn't find them in the scan bucket.
+            We couldn't find them in the scan bucket. We'll need to find out where the files are and kick them off
+            again or move them to technical failure.
 
-            Follow runbook to resolve:
-            https://github.com/alphagov/notifications-manuals/wiki/Support-Runbook#Deal-with-letter-pending-virus-scan-for-90-minutes.
             Notifications: {sorted(letter_ids)}"""
 
         if current_app.config['NOTIFY_ENVIRONMENT'] in ['live', 'production', 'test']: