Don't try to agree a MAC we'll never use.

Don't try to agree a MAC we'll never use.

If we've chosen the ChaCha20-Poly1305 option for a cipher, then that
forces the use of its associated MAC. In that situation, we should
avoid even _trying_ to figure out a MAC by examining the MAC string
from the server's KEXINIT, because we won't use the MAC selected by
that method anyway, so there's no point imposing the requirement on
servers to present a MAC we believe in just so we know it's there.

This was breaking interoperation with tinysshd, and is in violation of
OpenSSH's spec for the "chacha20-poly1305@openssh.com" cipher.