TransactionController: providerConfig -> selectedNetworkClientId (#4357)

The `providerConfig` state property is being removed from
NetworkController. Currently this property is used in
TransactionController to get the currently selected chain, but
`selectedNetworkClientId` can be used instead. This commit makes that
transition so that we can fully drop `providerConfig`.

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Co-authored-by: Jongsun Suh <jongsun.suh@icloud.com>

Author: 2 people