Remove i3-specific config

[chiarabellenghi]

https://github.com/icecube/skyllh/blob/d76a49c3c6176c6fb6fc13e14f51ce81d7f9cd73/skyllh/i3/config.py

This should be done within the general core/config

[chiarabellenghi]

This will probably be updated rather than removed.

The current idea is that this config will override the general one from SkyLLH for internal IceCube analyses.