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.