send_email: Change clear_scheduled_emails to only take one user.

No codepath except tests passes in more than one user_profile -- and
doing so is what makes the deduplication necessary.

Simplify the API by making it only take one user_profile id.

(cherry picked from commit ebaafb32f3)

zerver/lib/send_email.py

@@ -390,23 +390,20 @@ def clear_scheduled_invitation_emails(email: str) -> None:
 
 
 @transaction.atomic(savepoint=False)
-def clear_scheduled_emails(user_ids: List[int], email_type: Optional[int] = None) -> None:
+def clear_scheduled_emails(user_id: int, email_type: Optional[int] = None) -> None:
     # We need to obtain a FOR UPDATE lock on the selected rows to keep a concurrent
     # execution of this function (or something else) from deleting them before we access
     # the .users attribute.
-    items = ScheduledEmail.objects.filter(users__in=user_ids).select_for_update()
+    items = ScheduledEmail.objects.filter(users__in=[user_id]).select_for_update()
     if email_type is not None:
         items = items.filter(type=email_type)
 
-    deduplicated_items = {}
     for item in items:
-        deduplicated_items[item.id] = item
-    for item in deduplicated_items.values():
         # Now we want a FOR UPDATE lock on the item.users rows
         # to prevent a concurrent transaction from mutating them
         # simultaneously.
         item.users.all().select_for_update()
-        item.users.remove(*user_ids)
+        item.users.remove(user_id)
         if item.users.all().count() == 0:
             # Due to our transaction holding the row lock we have a guarantee
             # that the obtained COUNT is accurate, thus we can reliably use it