confirmation/views: Remove buggy behavior for expired confirmation links.

Previously, an expired preregistrationuser link would still be passed on to
/accounts/register (via the confirm_preregistrationuser.html template), just
with the PreregistrationUser.status not set to 1.

But accounts_register never checks prereg_user.status, and hence processes
the user as if the link had been confirmed.

With this commit, expired confirmation links never get past the confirmation
code.

confirmation/views.py

@@ -13,7 +13,7 @@ from django.http import HttpRequest, HttpResponse
 from confirmation.models import Confirmation
 from zerver.models import PreregistrationUser
 
-from typing import Any
+from typing import Any, Dict
 
 # This is currently only used for confirming PreregistrationUser.
 # Do not add other confirmation paths here.
@@ -21,16 +21,7 @@ def confirm(request, confirmation_key):
     # type: (HttpRequest, str) -> HttpResponse
     confirmation_key = confirmation_key.lower()
     obj = Confirmation.objects.confirm(confirmation_key)
-    confirmed = True
-    if not obj:
-        # confirmation failed
-        confirmed = False
-        try:
-            # try to get the object we was supposed to confirm
-            obj = Confirmation.objects.get(confirmation_key=confirmation_key)
-        except Confirmation.DoesNotExist:
-            pass
-    ctx = {'confirmed': confirmed}  # type: Dict[str, Any]
+    ctx = {'confirmed': False}  # type: Dict[str, Any]
     templates = [
         'confirmation/confirm.html',
     ]