[DOC] List of signing devices not up to date

[jp1ac4]

The information on signing devices here is out of date:
https://github.com/wizardsardine/liana/blob/master/doc/USAGE.md#signing-devices-and-hot-keys

I think it would be enough to remove the list of devices and keep only the link to the SIGNING_DEVICES.md page, which is kept up to date.

[pythcoiner]

agree, I always forgot to update it 😄

[nondiremanuel]

I agree with using a single source of truth in the docs to simplify the update procedure.