Remove curve25519-sha256@libssh.org for now

It did not make it in the last OpenSSH release,
we will re-add it with the next release, together with
chacha20-poly1305@openssh.com, ssh-ed25519,
ssh-ed25519-cert-v01@openssh.com and others.
Merge pull request #43 from ax3l/fix-openSSH64kex

Remove curve25519-sha256@libssh.org for now