Remove default target peer for proposer-only mode for PeerDAS.

[jimmygchen]

Issue Addressed

The default target peer for proposer node (=15) will need to be increased for peerdas. A node would need peers across all data column subnets to be able to publish the data columns successfully.

If we can have "decentralized blob building" working reliably on PeerDAS in the future, then proposers won't necessary need to publish blobs themselves and this number can potentially be reduced again.

[michaelsproul]

Please update to point at unstable by either:

1. Rebasing on unstable (if your branch has a small number of commits that are easy to tease out), or
2. Merging origin/das into this PR: git fetch origin; git merge origin/das. This will result in the smallest number of conflict resolutions and is better for branches that already contain merge commits or have extensive history.

[jimmygchen]

This feature makes the proposer-only mode less useful than it was. I'm inclined to see whether decentralized blob building #6268 formally make it into the spec before deciding to merge this one.

IMO I think we should ship peerdas with #6268, and we'd be able to keep the proposer-only feature.