removing php-cloudfiles, replaced by php-opencloud

This pull request removes php-cloudfiles from the 3rd party repository. Can be applied after applying https://github.com/owncloud/core/pull/5468.

php-cloudfiles is only used in the old integration of Swift. The new integration (https://github.com/owncloud/core/pull/5468) will use php-opencloud, included in the 3rd party directory of the files_external application.