* writeJson: drop unused JSONP branch
No in-tree caller uses ?callback=. Always serve application/json
with X-Content-Type-Options: nosniff.
* seaweed-volume: drop unused JSONP branch
Mirror Go: always serve application/json with
X-Content-Type-Options: nosniff.
* writeJson: drop unreachable StatusNotModified check
bodyAllowedForStatus already returns early for 304.
* test/volume_server: rename and rewrite JSONP test to assert callback is ignored
CI: /status?callback=myFunc now returns plain application/json
with X-Content-Type-Options: nosniff.