Port EWS away from QCA

QCA is being sunset (libraries/qca#18). EWS uses it for public-key based authentication, which we don't know whether it is actually being used actively as we assume that majority of users will be using the OAuth-based stuff.

We think that it should be possible to just remove (comment out/#ifdef-out) the entire public-key authentication and see if anyone complains that it broke for them.