Remove private_keys from Signer

[enricobottazzi]

summa-solvency/backend/src/contracts/signer.rs

Line 33 in f5d04b1

private_keys: &[&str],

Given the latest discussions, I propose removing the support for private_keys from the signer struct as any Exchange interacting with Summa will provide us a file containing the signature data and it's very unlikely that they will use our APIs to perform the signatures.

This modification will also help trimming lines of code and making our backend more lean.