gas_price parameter still mentioned in the error message

[akolotov]

After changes made under https://github.com/poanetwork/poa-bridge/pull/80 gas price for bridge transactions is fetched from the oracle so there is no need to specify it in transactions section. But this parameter still exists in the bridge data structures
https://github.com/poanetwork/poa-bridge/blob/13f78f5aad4a934c544cad21f6569dde90777f78/bridge/src/config.rs#L205-L212

So, if an incorrect parameter is specified in transactions section of the configuration file the error message will suggest to specify gas_price:

unknown field `some_parameter`, expected `gas` or `gas_price` for key `transactions.deposit_relay`