readme: remove confusion
readme: remove confusion

`publicKey` is a remnant from back when ssh2
didn't have the ability to derive the public key
from the private key.

Drooids/ssh2,Student007/ssh2,mcanthony/ssh2,24hr/ssh2,onilabs/ssh2,mscdex/ssh2,jskirst/sftp-to-http,ianscrivener/ssh2,mcanthony/ssh2,Drooids/ssh2,jskirst/sftp-to-http,mscdex/ssh2,24hr/ssh2,Student007/ssh2,ArielAbreu/ssh2,mscdex/ssh2,ArielAbreu/ssh2,mscdex/ssh2,onilabs/ssh2,ianscrivener/ssh2