settings: Remove unused setting ACCOUNT_ACTIVATION_DAYS.

ACCOUNT_ACTIVATION_DAYS doesn't seems to be used anywhere.
INVITATION_LINK_VALIDITY_DAYS seems to do it's job currently.

(It was only ever used in very early Zulip commits).

zerver/lib/actions.py

@@ -4887,7 +4887,7 @@ def do_invite_users(user_profile: UserProfile,
     notify_invites_changed(user_profile)
 
 def do_get_user_invites(user_profile: UserProfile) -> List[Dict[str, Any]]:
-    days_to_activate = getattr(settings, 'ACCOUNT_ACTIVATION_DAYS', 7)
+    days_to_activate = settings.INVITATION_LINK_VALIDITY_DAYS
     active_value = getattr(confirmation_settings, 'STATUS_ACTIVE', 1)
 
     lowest_datetime = timezone_now() - datetime.timedelta(days=days_to_activate)

zerver/tests/test_signup.py

@@ -1255,7 +1255,7 @@ class InvitationsTestCase(InviteUserBase):
         A GET call to /json/invites returns all unexpired invitations.
         """
 
-        days_to_activate = getattr(settings, 'ACCOUNT_ACTIVATION_DAYS', "Wrong")
+        days_to_activate = getattr(settings, 'INVITATION_LINK_VALIDITY_DAYS', "Wrong")
         active_value = getattr(confirmation_settings, 'STATUS_ACTIVE', "Wrong")
         self.assertNotEqual(days_to_activate, "Wrong")
         self.assertNotEqual(active_value, "Wrong")

zproject/settings.py

@@ -1267,8 +1267,6 @@ LOGGING = {
     }
 }
 
-ACCOUNT_ACTIVATION_DAYS = 7
-
 LOGIN_REDIRECT_URL = '/'
 
 # Client-side polling timeout for get_events, in milliseconds.