refactor: remove `isseednode`

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Issue being fixed or feature implemented

isseednode no longer served any purpose, because seed nodes are configured by dashmate. Additionally, all non-seed nodes were pointing at the same seed node (always seed-1).

What was done?

• Remove isseednode
• Target seed nodes evenly when templating dash.conf

How Has This Been Tested?

On testnet

Breaking Changes

None

Checklist:

• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have added or updated relevant unit/integration/functional/e2e tests
• [x] I have made corresponding changes to the documentation

For repository code-owners and collaborators only

• [x] I have assigned this pull request to a milestone

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Actually I might have over-complicated this. dashmate just templates in all available seeds, maybe we should do the same here?