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

This reverts commit a927709a because it
doesn't build:

$ nix-build -A openssh
...
mkdir /nix/store/yl2xap8n1by3dqxgc4rmrc4s753676a3-openssh-7.2p2/libexec
(umask 022 ; ./mkinstalldirs /var/empty)
mkdir /var
mkdir: cannot create directory '/var': Permission denied
mkdir /var/empty
mkdir: cannot create directory '/var/empty': No such file or directory
make: *** [Makefile:304: install-files] Error 1
builder for ‘/nix/store/ifygp4mqpv7l8cgp0njp8w7lmrl6brpp-openssh-7.2p2.drv’ failed with exit code 2