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
Urp, nevermind me. Those are packaged virtualenvs. I'll keep digging.