Remove legacy global/selected-chain path from GasFeeController

[sirtimid]

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.