groups: Do not prefetch can_access_all_users_group setting.

This commit updates code to not prefetch can_access_all_users_group
and can_access_all_users_group__named_user_group fields using
select_related. We can just use get_realm_system_groups_name_dict
function to check if setting is set to "Everyone" group when
needed and can avoid unnecessarily fetching groups for every user
query.

zerver/tests/test_signup.py

@@ -1028,8 +1028,8 @@ class LoginTest(ZulipTestCase):
         # to sending messages, such as getting the welcome bot, looking up
         # the alert words for a realm, etc.
         with (
-            self.assert_database_query_count(94),
-            self.assert_memcached_count(15),
+            self.assert_database_query_count(93),
+            self.assert_memcached_count(18),
             self.captureOnCommitCallbacks(execute=True),
         ):
             self.register(self.nonreg_email("test"), "test")