Fix or remove Covalent / add a token list

[evan-gray]

The covalent related code likely doesn't work correctly in the current testnet environment (and the key isn't set in the current deploy). Either the chains which covalent supports for testnet should be used and the rest appropriately omitted from querying, or Covalent support should just be omitted. It might be nicer to have a dedicated set of supported tokens for testnet and devnet anyway.