TokenRatesController: providerConfig -> selectedNetworkClientId (#4317)

The `providerConfig` state property is being removed from
NetworkController. Currently this property is used in
TokenRatesController to get the currently selected chain and ticker, but
`selectedNetworkClientId` can be used instead to get the currently
selected network client, and then the appropriate information can be
read from that object. This commit makes that transition so that we can
fully drop `providerConfig`.

Author: mcmire

packages/assets-controllers/CHANGELOG.md

@@ -14,6 +14,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - This should be functionally equivalent, but is being noted anyway.
 - NftDetectionController now makes use of `selectedNetworkClientId` when responding to changes in NetworkController state to capture the currently selected chain rather than `providerConfig` ([#4307](https://github.com/MetaMask/core/pull/4307))
   - This should be functionally equivalent, but is being noted anyway.
+- TokenRatesController now makes use of `selectedNetworkClientId` when responding to changes in NetworkController state to capture the currently selected chain rather than `providerConfig` ([#4317](https://github.com/MetaMask/core/pull/4317))
+  - This should be functionally equivalent, but is being noted anyway.
 
 ## [29.0.0]