channel: Remove idempotent retry loop.

This was added by commit 7f174213ed, and
appears to have been designed for responses that are *successful* but
falsy. Logically, these should not implicitly represent a failure to
be retried if it were.

Note from tabbott: The background is that this idempotent retry loop
was a hacky workaround for a bug we never understood but saw daily in
production. Especially during server restarts / client reloads,
something would result in 200 responses with no data being seen by the
frontend, despite the Django server not having received/processed the
request. Fortunately, this strange failure mode appears to have
stopped happening in late 2019, so we can delete this hack.

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

static/js/compose.js

@@ -381,7 +381,6 @@ export function render_and_show_preview($preview_spinner, $preview_content_box,
         }
         channel.post({
             url: "/json/messages/render",
-            idempotent: true,
             data: {content},
             success(response_data) {
                 if (markdown.contains_backend_only_syntax(content)) {