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/message_fetch.js

@@ -261,7 +261,6 @@ export function load_messages(opts) {
     channel.get({
         url: "/json/messages",
         data,
-        idempotent: true,
         success(data) {
             get_messages_success(data, opts);
         },