compose_validate: Remove autosubscribe feature.

This was only used in the undocumented narrow_stream mode, and relied
on a deprecated synchronous XHR request.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zproject/urls.py

@@ -234,7 +234,6 @@ from zerver.views.video_calls import (
 )
 from zerver.views.zephyr import webathena_kerberos_login
 from zproject import dev_urls
-from zproject.legacy_urls import legacy_urls
 
 if settings.TWO_FACTOR_AUTHENTICATION_ENABLED:  # nocoverage
     from two_factor.gateways.twilio.urls import urlpatterns as tf_twilio_urls
@@ -242,9 +241,6 @@ if settings.TWO_FACTOR_AUTHENTICATION_ENABLED:  # nocoverage
 
 # NB: There are several other pieces of code which route requests by URL:
 #
-#   - legacy_urls.py contains API endpoint written before the redesign
-#     and should not be added to.
-#
 #   - runtornado.py has its own URL list for Tornado views.  See the
 #     invocation of web.Application in that file.
 #
@@ -866,4 +862,4 @@ urls += [path("health", health)]
 # The sequence is important; if i18n URLs don't come first then
 # reverse URL mapping points to i18n URLs which causes the frontend
 # tests to fail
-urlpatterns = i18n_patterns(*i18n_urls) + urls + legacy_urls
+urlpatterns = i18n_patterns(*i18n_urls) + urls