openssh: Use the default privilege separation dir (/var/empty)

(This is a rewritten version of the reverted commit
a927709a, that disables the creation of
/var/empty during build so that sandboxed builds also works. For more
context, see https://github.com/NixOS/nixpkgs/pull/16966)

If running NixOS inside a container where the host's root-owned files
and directories have been mapped to some other uid (like nobody), the
ssh daemon fails to start, producing this error message:

fatal: /nix/store/...-openssh-7.2p2/empty must be owned by root and not group or world-writable.

The reason for this is that when openssh is built, we explicitly set
`--with-privsep-path=$out/empty`. This commit removes that flag which
causes the default directory /var/empty to be used instead. Since NixOS'
activation script correctly sets up that directory, the ssh daemon now
also works within containers that have a non-root-owned nix store.