Revisit the usefulness of note erasure at block level

[mmagician]

I think I'm skeptical of the usefulness of note erasure at block level, given that batches are already incentivized to do as much note erasure as possible, and it just adds extra complexity.

Originally posted by @PhilippGackstatter in #2904 (comment)

[bobbinth]

I think this is not a high priority issue either way. That is, we can start w/o note erasure at the block level, and the introduce it later in a relatively painless way.