feat(governance): remove pre-Conway SPO poll tests and utils

Remove legacy governance poll tests and related data files, including:
- test_governance.py with all pre-Conway poll test cases
- poll_utils.py with helper functions for poll creation, answering, and verification
- golden_stake_pool.skey and governance_poll.json test data

These files are no longer needed as the governance poll functionality has
been deprecated in favor of the Conway on chain governance.

Author: mkoura