[Minor] WebUI: drop unreachable 304 success branch

[Minor] WebUI: drop unreachable 304 success branch

The Rspamd controller sends "Cache-Control: no-store" on every WebUI
response (controller.c, issue #3330) and never emits Last-Modified/ETag
for these endpoints, so a browser never revalidates and a 304 response
is impossible. The `|| xhr.status === 304` in queryServer was dead code
copied from jQuery's default success range.

Remove it so queryServer, the /stat connect probe and the legacy /auth
probe all treat 200-299 as success. Treating 304 as an error is also
safer: a 304 body is empty, and success would have fed it to the JSON
parser.