Remove DA header from proposal

[adlerjohn]

Pending acceptance of ADR-006, the PartSetHeader is used to gossip blocks in the consensus reactor, which means there is no need to include the DA header in the proposal since validators don't do DAS.

As per celestiaorg/celestia-specs#180 (comment), consider removing the DA header from proposal, if we use PartSetHeader to gossip blocks.

DAHeader is included in proposal: if validators don't do DAS this is not a strict requirement anymore! Only if the spec wants to very vaguely and very implicitly make assumptions around block propagation during consensus, which indeed is an implementation detail. If we want to keep the DAHeader in the Proposal, we should clarify here why DAHeader is included in the first place (previously for sampling, now for ??). Also if proposal include the DAHeader why don't Votes also include it?

[evan-forbes]

completed in #518

and since this reversion puts on par with tendermint, we should simply point to the tendermint specs