Enable "SSH Keys" auth panel unconditionally

Summary: We've had support for this for a long time, but it was conditional on config. Since it more-or-less actually does something now, just enable it unconditionally.

Test Plan: Settings -> SSH Public Keys

Reviewers: btrahan

Reviewed By: btrahan

CC: hach-que, aran

Maniphest Tasks: T2230

Differential Revision: https://secure.phabricator.com/D7426