[GNS] Refactor `NFTController`: Remove Private Property `#chainId` and the fallback on current selected network

In `NFTController`, the private property `#chainId` currently falls back to the `selectedNetwork`, as seen in the `#onNetworkControllerNetworkDidChange` method. This property is no longer needed with the remove of GNS

### 🎯 Goal

- Remove the `#chainId` private property entirely
- Refactor all functions that use `#chainId` to **require `chainId` as an explicit, mandatory parameter**

