test_urls: Remove legacy get_callback_string.
test_urls: Remove legacy get_callback_string.

`_callback_str` was removed in Django in 1.10, and other logic relevant
to that particular attribute was removed in
32849b80ad96ce898c4aab10b1029b398c353c99, but not to its entirety. It
does not make sense to fall back to `_callback_str`. The
`get_callback_string` helper is no longer needed.

Signed-off-by: Zixuan James Li <74923aee05c23ca90566381b5d6b0cd9451b6054@gmail.com>

rht/zulip,zulip/zulip,andersk/zulip,andersk/zulip,rht/zulip,rht/zulip,rht/zulip,andersk/zulip,andersk/zulip,rht/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,rht/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip