internal/ethapi: don't set sender-balance to maxuint, fixes #16999 (#20783)

Prior to this change, eth_call changed the balance of the sender account in the
EVM environment to 2^256 wei to cover the gas cost of the call execution.
We've had this behavior for a long time even though it's super confusing.

This commit sets the default call gasprice to zero instead of updating the balance,
which is better because it makes eth_call semantics less surprising. Removing
the built-in balance assignment also makes balance overrides work as expected.

internal/ethapi/api.go

@@ -47,10 +47,6 @@ import (
 	"github.com/tyler-smith/go-bip39"
 )
 
-const (
-	defaultGasPrice = params.GWei
-)
-
 // PublicEthereumAPI provides an API to access Ethereum related information.
 // It offers only methods that operate on public data that is freely available to anyone.
 type PublicEthereumAPI struct {
@@ -808,7 +804,7 @@ func DoCall(ctx context.Context, b Backend, args CallArgs, blockNrOrHash rpc.Blo
 		log.Warn("Caller gas above allowance, capping", "requested", gas, "cap", globalGasCap)
 		gas = globalGasCap.Uint64()
 	}
-	gasPrice := new(big.Int).SetUint64(defaultGasPrice)
+	gasPrice := new(big.Int)
 	if args.GasPrice != nil {
 		gasPrice = args.GasPrice.ToInt()
 	}