Remove placeholder in Liberty directory
Remove placeholder in Liberty directory

Now that we have our first Liberty spec, we don't need this
placeholder.rst.

Change-Id: I5ceff1271a686f1d7fb5ab74261f9ab7eacb6f2b

NaohiroTamura/ironic-specs,naterh/ironic-specs,bcornec/ironic-specs,openstack/ironic-specs