Disable "high performance ssh" openssh patch Gentoo includes

Prioritizing security and stability over performance in SSH, omitting
this kind of patch is generally more consistent with our objectives.

Visibly this removes "p1-hpn14v4" from the OpenSSH protocol banner:
SSH-2.0-OpenSSH_6.6p1-hpn14v4

Discussion: https://github.com/coreos/bugs/issues/149