Update NLP Provider List

[xeruf]

LocalWifiNlpBackend has been superseded by DejaVu, which itself has been forked by https://github.com/Helium314/Local-NLP-Backend - consider linking one of the newer ones.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

[Nichtraucher]

I believe it refers to the WiFi emitter data that is also being collected but cannot be provided directly as a download because of legal reasons (privacy protection).