remove special handling of ipython_kernel in launcher

- set JPY_PARENT_PID and JPY_INTERRUPT_EVENT env variables for all kernels
- no custom kernel args other that console passthrough
- use devnull instead of `--no-stdout` (comments claim it shouldn't work, but tests suggest it does, so let's keep an eye on this)

closes #7085
This is definitely not an easy one to deprecate.

A [quick search on GitHub shows 51 code results](https://github.com/search?q=IPY_INTERRUPT_EVENT&type=Code&utf8=%E2%9C%93) using this environment variable. After the release we can make PRs against all of those repos, but it doesn't fix it for any private usage.