Remove providerConfig from NetworkController

[mcmire]

Historically, the providerConfig property in NetworkController has been used to switch and track the currently selected network as well as provide access to information about that network. That is now achieved with a combination of the selectedNetworkClientId and networkConfigurations properties as well as the configuration property on the NetworkClient interface. This means that we no longer need providerConfig and we can remove this redundant state.

Removing providerConfig is one thing, but there are also a ton of packages which make use of this property to access the currently selected chain, and we would want to make use of selectedNetworkClientId in combination with the NetworkController:getNetworkClientById action to do this instead.

Acceptance Criteria

• providerConfig is removed from NetworkController (replaced with selectedNetworkClientId and/or `networkConfigurations); NetworkController tests pass
• [assets-controllers] AccountTrackerController is updated to follow suit and tests pass
• [assets-controllers] AssetsContractController is updated to follow suit and tests pass
• [assets-controllers] NftController is updated to follow suit and tests pass
• [assets-controllers] NftDetectionController is updated to follow suit and tests pass
• [assets-controllers] TokensController is updated to follow suit and tests pass
• [assets-controllers] TokenListController is updated to follow suit and tests pass
• [assets-controllers] TokenRatesController is updated to follow suit and tests pass
• EnsController is updated to follow suit and tests pass
• GasFeeController is updated to follow suit and tests pass
• SelectedNetworkController tests are updated to follow suit and tests pass
• TransactionController is updated to follow suit and tests pass