Tahoe: Avoid verifying hash after download, since tahoe does sufficient verification...

Tahoe: Avoid verifying hash after download, since tahoe does sufficient verification itself

See my comment in the next commit for some details about why
Verified needs a hash with preimage resistance. As far as tahoe goes,
it's fully cryptographically secure.

I think that bup could also return Verified. However, the Retriever
interface does not currenly support that.