nginx: Remove invalid extra headers for OPTIONS /api/v1/events.

Since 204 responses don’t contain a payload body, Content-Type is
neither required nor encouraged (RFC 7231 §3.1.1.5), and ours was
missing a semicolon to boot; Content-Length is expressly
forbidden (RFC 7230 §3.3.2).

Furthermore, these add_header directives were silencing the CORS
headers set in api_headers, because add_header inheritance doesn’t
work the way you think it does, as was known before commit
5614d51afc.

Fixes: #12902.

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

puppet/zulip/files/nginx/zulip-include-frontend/app

@@ -30,8 +30,6 @@ location /api/v1/events {
     include /etc/nginx/zulip-include/api_headers;
 
     if ($request_method = 'OPTIONS') {
-        add_header 'Content-Type' 'text/plain charset=UTF-8';
-        add_header 'Content-Length' 0;
         return 204;
     }