TokenRatesController Technical Debt

[kanthesha]

The TokenRatesController state which is mapping of token address and exchange rate used to be managed with ContractExchangeRates type. But recently, it's completely modified and new ContractMarketData has been added to manage it's state. Now there's no need of ContractExchangeRates and needs to be removed.

This ticket, requires to analyse if there is still ContractExchangeRates used where the ContractMarketData build included. And remove all it's occurrences and also rename the variables and functions which are still pre-fixed / post-fixed with ContractExchangeRate.