integrations_dev_panel: Remove unused idempotent flag.

Followup to commit fde9b1d366 (#22753).

(This was a misuse of “idempotent”.  “Idempotent” means that
performing the request more than once in a row has the same effect as
performing it once; it says nothing about whether the response is
cacheable.)

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

static/js/portico/integrations_dev_panel.js

@@ -223,8 +223,6 @@ function get_fixtures(integration_name) {
     // /devtools/integrations/<integration_name>/fixtures
     channel.get({
         url: "/devtools/integrations/" + integration_name + "/fixtures",
-        // Since the user may add or modify fixtures as they edit.
-        idempotent: false,
         success(response) {
             loaded_fixtures.set(integration_name, response.fixtures);
             load_fixture_options(integration_name);