Remove empty __init__.py files
Especially the main one in swh/objstorage needs to be removed since this later is a namespace, in addition to being a package, in which swh-objstorage-replayer installs itself. So for the namespace importing mechanism to work properly, it must be an actual namespace (handled by the NamespaceLoader, not a package backed by the init file).
(for some reason this used to work but seems now broken; haven't dug)