Remove legacy polling code from TokenRatesController

There are two polling strategies represented in TokenRatesController:

1. The original strategy, which always fetches rates for the globally selected network. To use this, you call `start` to start the loop and `stop` to stop the loop. The loop itself is implemented via `#poll`.
2. The new strategy as represented via the PollingController, which allows for fetching rates for arbitrary networks. To use this, you start a new loop by using `startPollingByNetworkClientId`, passing the ID of a network client; you can also stop an existing loop via `stopPollingByPollingToken`, or stop all loops by calling `stopAllPolling`. Internally, a loop is implemented via `_executePoll`.

Maintaining the original polling strategy costs time and effort, as we have to update the tests associated with the methods mentioned above every time we update TokenRatesController. We can remove them and ask clients to use the new polling API.

Start by locating TokenRatesController and the tests for start, stop, and poll, then compare them with PollingController and its _executePoll implementation. Remove the legacy polling strategy and update the affected tests and clients to use startPollingByNetworkClientId, stopPollingByPollingToken, or stopAllPolling; done means the old API and its tests are gone.

Written by the indexing model from the issue text.