Always use XDG config directory on Linux

We've put config files in `.config/ipython` on Linux by default for a few releases, and I think it's time to force the change. This simplifies what we have to tell people about where their config files are.

Before:

- On Windows or Mac, it's in `.ipython`
- On Linux, it's in:
  - `.config/ipython` if `.ipython` doesn't exist
  - `.ipython` if that exists and `.config/ipython` doesn't
  - `.config/ipython` if that and `.ipython` both exist

After:

- On Windows or Mac, it's in `.ipython`
- On Linux, it's in `.config/ipython`