Remove the Rackspace Cloud Files backend

[meeb]

The Rackspace Cloud Files backend currently depends on Pyrax, which is Python2 only and depreciated. While there is an apparent replacement in the form of rackspacesdk all documentation is still for Pyrax and has not been updated. It's quite likely no-one uses this backend anyway any more given the popularity of S3 and Google Cloud Storage.

General plan at the moment is to remove the Rackspace Cloud Files backend and if anyone complains re-implement it with the openstacksdk, likely at quite some time expense given the sparse documentation and awkward testing.