pull: now that PullJob can verify expected digests, let's rely on it for tar/raw...

pull: now that PullJob can verify expected digests, let's rely on it for tar/raw pulling

Instead of authenticating the downloaded image explicity in the tar and
in the raw downloader, we can now rely on the checksum checking in the
generic PullJob code. Hence do so: drop tep the checksum field from
TarPull and RawPull, and just initialized the ->expected_checksum in the
relevant PullJob instead.