chore: remove deprecated senate/Triumvirate dead code

[cyclone-pro]

Bug

No existing issue — senate/Triumvirate pallet was silently removed from the subtensor
runtime. The orphaned code remained with TODO comments questioning its status:

# TODO: is this all deprecated? Didn't subtensor senate stuff get removed?

Description of the Change

Removes all dead code related to the senate/Triumvirate voting system that was left
behind after the pallet was dropped from the subtensor runtime.

Files removed or cleaned up:

• bittensor/core/chain_data/proposal_vote_data.py — deleted entirely
• bittensor/core/subtensor.py — removed get_vote_data() method and import
• bittensor/core/async_subtensor.py — removed async get_vote_data() and import
• bittensor/core/chain_data/__init__.py — removed ProposalVoteData export
• bittensor/utils/easy_imports.py — removed ProposalVoteData from public API
• bittensor/extras/subtensor_api/chain.py — removed get_vote_data binding
• tests/unit_tests/test_async_subtensor.py — removed 3 senate-related unit tests
• tests/e2e_tests/test_delegate.py — removed dead import (e2e senate tests were
  already commented out)

Evidence that senate was removed from the runtime:

• Proxy enum entries (Senate, Triumvirate, Governance) were already marked
  # deprecated proxy types in chain_data/proxy.py
• All e2e tests for senate voting in test_delegate.py were already commented out
• No senate extrinsics or commands exist anywhere in the codebase
• No mentions of senate in CHANGELOG for v10.x
• The method queried the Triumvirate pallet's Voting storage — a pallet that no
  longer exists on the active chain

Alternate Designs

Considered adding a DeprecationWarning to get_vote_data() instead of deleting it.
Chose full deletion because the underlying pallet no longer exists on-chain — any call
to this method was already silently returning None or raising a substrate error.
Keeping it would give users false confidence that the API is functional.

Possible Drawbacks

Any external code calling subtensor.get_vote_data() will get an AttributeError
after upgrading. However, since the Triumvirate pallet is gone from the runtime, those
calls were already broken at the chain level. The behavioral change is: fail loudly at
import/call time instead of silently returning None.

Verification Process

1. Searched the full codebase for all references to ProposalVoteData, get_vote_data,
   and Triumvirate — confirmed none remain in production code.
2. Ran the full async subtensor unit test suite:
   pytest tests/unit_tests/test_async_subtensor.py -v

Result: 216 passed, 0 failed, 1 warning (the warning is pre-existing and unrelated).

Release Notes

Removed the defunct senate/Triumvirate voting API (get_vote_data, ProposalVoteData)
that was no longer functional after the senate pallet was removed from the subtensor runtime.

Branch Acknowledgement

[x] I am acknowledging that I am opening this branch against staging

[cyclone-pro]

CC: @basfroman - would appreciate a review when you get a chance.
This removes the orphaned senate/Triumvirate code that was left
after the pallet was dropped from subtensor runtime.

[thewhaleking]

I'm not going to approve or request changes, I'll leave that decision up to @basfroman, but with that being said, I err on the side of not removing dead code in Bittensor until it's "stone dead", because dead things in this ecosystem tend to crop back up with some regularity.