Delete node mimir

When nodes vote, they can now set their vote to -1 to delete their vote (if present). This allows a vote to go from a stance of opinioned to not (omit)

Summary by CodeRabbit

• Bug Fixes
  • Improved handling of node mimir records so that setting a negative value now correctly removes the corresponding record.
• Tests
  • Added new test scenarios verifying node vote removals and mimir key deletion.
  • Updated regression test data to include transactions setting mimir values to negative.
  • Refreshed test fixtures with updated account sequences, balances, and network parameters.