(stable) Remove the "ssh-auth-key" script

Summary:
Ref T13436. Historically, this script could be used with a forked copy of "sshd" to do lower-cost per-key auth.

Relatively modern "sshd" supports "%f" to "AuthorizedKeysCommand", which effectively moots this.

Users have never been instructed to use this script for anything, and we moved away from this specific patch to "sshd" some time ago.

Test Plan: Grepped for "ssh-auth-key", no hits.

Maniphest Tasks: T13436

Differential Revision: https://secure.phabricator.com/D20873