Remove data persistence layer since #1279018 is fixed

[Andreas Hasenack]

Bug #1279018 has been fixed for a long time. Looks like we can remove the persistence layer we added as a workaround:
# XXX We can remove _persist_data when juju bug:1279018 is fixed to provide
# relation-data from *-relation-departed hook runs
def _persist_data(key, value):
    """Save C{key} and C{value} pairs in a persistent file for reference"""
    data = {}
    filepath = "%s/%s" % (hookenv.charm_dir(), PERSIST_DATA_FILE)
    if os.path.exists(filepath):
        with open(filepath) as inputfile:
            data = json.load(inputfile)
    data[key] = value
    with open(filepath, "w") as outfile:
        outfile.write(unicode(json.dumps(data, ensure_ascii=False)))