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.

@azet it seems this change, as discussed in the mailing list, did not make it in yet (@aaronkaplan compiled the lastest version just now).