[Assets Controllers] Remove old polling APIs

- TokenDetectionController
- TokenListController
- TokenRatesController

Still have the old polling API with `start()` and `stop()` functions. These should no longer be used, and safe to remove. The `startPolling(input)` and `stopPollingByPollingToken(pollingToken)` functions from the base class `StaticIntervalPollingController` should be used instead.

Start with TokenDetectionController, TokenListController, and TokenRatesController, then inspect their inherited StaticIntervalPollingController APIs. Find the old start() and stop() methods and their references, and confirm that polling uses startPolling(input) and stopPollingByPollingToken(pollingToken) instead. Done means the old polling APIs are removed from all three controllers without remaining references.

Written by the indexing model from the issue text.