Remove legacy global/selected-chain path from GasFeeController

## Summary

`GasFeeController` supports two modes: the multichain path (`fetchGasFeeEstimates({ networkClientId })` → `gasFeeEstimatesByChainId[chainId]`) and a legacy **global/selected-chain** path that predates multichain support. Remove the global path so the per-`networkClientId` route is the only one.

## Surface to remove

- Constructor options: `getProvider`, `getChainId`, `getCurrentNetworkEIP1559Compatibility`, `getCurrentNetworkLegacyGasAPICompatibility`, `getCurrentAccountEIP1559Compatibility`, `onNetworkDidChange`
- Flat top-level state: `gasFeeEstimates`, `gasEstimateType`, `estimatedGasFeeTimeBounds`
- No-arg polling `getGasFeeEstimatesAndStartPolling()` and the `NetworkController:networkDidChange` subscription/reset
- Make `networkClientId` required in `fetchGasFeeEstimates`; give `getTimeEstimate` a chain/`networkClientId` arg

## Impact — **BREAKING**

- **Core:** low risk. No consumer reads the flat state or uses no-arg polling; transaction-controller already passes `networkClientId`. Mostly mechanical + large `GasFeeController.test.ts` rewrite (~60-70% is global-path).

## Notes

Follow-up to #9569, which made the global path init-order-agnostic — this removal deletes that code path entirely.

Start with GasFeeController and the large GasFeeController.test.ts suite, separating the multichain path from the legacy global/selected-chain cases listed in the issue. Check transaction-controller's existing networkClientId calls, then remove the legacy surface and update the tests so the required per-networkClientId flow remains covered.

Written by the indexing model from the issue text.