ssh: remove note about local change to [Use]PrivilegeSeparation

We documented "[Use]PrivilegeSeparation defaults to sandbox" as one of
our modifications to ssh's server-side defaults, but this is not (any
longer) a difference from upstream.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit a752e011a862f9256b8a3b36d73ee1bbbcaf7a9b)