Remove BitcoinAverage and CoinMarketCap feeds when safe to do so.

[ghost]

Neither BitcoinAverage nor CoinMarketCap are used anymore as price sources. But dummy price providers from them insert timestamps into the pricenode feed. The Bisq client had a legacy dependency expecting timestamps from BitcoinAverage, Poloniex, and CoinMarketCap. The dependency was removed in bisq-network/bisq#6822.

Once Bisq has upgraded protocol requiring all clients upgrade, or enough time has passed, it will be safe to remove BitcoinAverage and CoinMarketCap classes from pricenode providers.

This issue is a reminder to do that cleanup when the time comes.