Update NLP Provider List

[LocalWifiNlpBackend](https://github.com/n76/wifi_backend) has been superseded by DejaVu, which itself has been forked by https://github.com/Helium314/Local-NLP-Backend - consider linking one of the newer ones.

Since OpenWLANMap and [LocalGSMLocationProvider](https://github.com/rtreffer/LocalGSMLocationProvider) are heavily dated, I'd suggest omitting them from the readme, to prevent information overload.

What do you mean by
> Only the cell tower database is free.

for the mozilla nlp?

Start by locating the README section listing NLP providers. Review the links and descriptions against the issue: link a newer Local NLP backend, omit the dated OpenWLANMap and LocalGSMLocationProvider entries, and clarify what the Mozilla NLP statement means. The issue has no recent discussion, so confirm the intended wording before editing.

Written by the indexing model from the issue text.