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

[salimtb]

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