Westend RC: remove governance post AHM

[sigurpol]

TL;DR

As followup of #11763 / #11705, we should also remove governance and opengov stack from Westend RC - not needed and used anymore post-AHM.

This follows up on #11705 which removed pallet_treasury and the Treasurer track from the relay.

What to remove

A draft list of changes to be performed below - to be double checked carefully.

From polkadot/runtime/westend/src/lib.rs construct_runtime!:

• pallet_conviction_voting (idx 31)
• pallet_referenda (idx 32)
• pallet_custom_origins (idx 35) — including all remaining origin variants:
  StakingAdmin, GeneralAdmin, FellowshipAdmin, AuctionAdmin, LeaseAdmin,
  ReferendumCanceller, ReferendumKiller, SmallTipper, BigTipper, SmallSpender,
  MediumSpender, BigSpender, WhitelistedCaller, and the various FellowshipN-Dan
  variants
• pallet_whitelist (idx 36)

Pallet indices 31, 32, 35, 36 become permanently unused (same treatment as idx 37 in #11705).

Cascading consumers to re-root or remove:

• Every type X = EitherOf<EnsureRoot, Y> where Y is a governance
  track origin (grep in polkadot/runtime/westend/src/lib.rs for
  EitherOf<EnsureRoot, AuctionAdmin, GeneralAdmin, LeaseAdmin, StakingAdmin,
  etc.) must be re-rooted to plain EnsureRoot.
• polkadot/runtime/westend/src/governance/tracks.rs: delete file entirely.
• polkadot/runtime/westend/src/governance/origins.rs: delete file entirely.
• polkadot/runtime/westend/src/governance/mod.rs: delete module; remove all
  re-exports from lib.rs.
• polkadot/runtime/westend/src/xcm_config.rs: remove *ToPlurality type aliases
  (GeneralAdminToPlurality, StakingAdminToPlurality,
  FellowshipAdminToPlurality), their *BodyId parameter_types, and drop
  LocalPalletOriginToLocation entirely. Rewire SendXcmOrigin to use only
  LocalOriginToLocation (or a direct EnsureRoot-based path).
• pallet_referenda::Config::Slash =
  pallet_dap_satellite::DapSatelliteLegacyAdapter<Runtime, Balances> — goes away
  with the pallet.

System parachain cleanup (follow-on opportunity)

Once the relay has no plurality-producing origins, ParentOrParentsPlurality matchers on all 5 Westend system parachain ' AllowExplicitUnpaidExecutionFrom barriers become dead (the (1, [Plurality { .. }]) arm can never match). Can be simplified to a plain parent matcher.

Extra notes

• Add frame_support::migrations::RemovePallet migrations for each pallet to clear orphaned storage (same pattern as the RemovePallet<"Treasury", _> in Remove RC and local treasury accounts #11705).
• must-do: try-runtime dry-run against live Westend

Related issues

• Remove RC and local treasury accounts #11705 treasury + Treasurer removal (fixed by [westend] Remove pallet_treasury from RC and clean up system parachains matchers #11763 )
• Add token transfers via XCM to DAP from the satellites #11434 relay → AH teleport for DAP funds (orthogonal but related AHM cleanup)

Fellowship runtime

• Polkadot / Kusama: related issue Remove old OpenGov configuration from the polkadot relay polkadot-fellows/runtimes#1102