Remove optional dependency on swh.objstorage.cloud

If you want the archiver to have cloud abilities, install the
package python3-swh.objstorage.cloud on the server as well.

requirements.txt

@@ -11,7 +11,6 @@ flask
 # Internal dependencies
 swh.core >= 0.0.23
 swh.objstorage >= 0.0.2
-swh.objstorage.cloud >= 0.0.9
 
 click
 swh.scheduler