core: use max to simplify function CalcGasLimit (#1902)

Author: wit765

core/block_validator.go

@@ -189,10 +189,7 @@ func CalcGasLimit(parent *types.Block) uint64 {
 		at that usage) the amount increased/decreased depends on how far away
 		from parentGasLimit * (2/3) parentGasUsed is.
 	*/
-	limit := parent.GasLimit() - decay + contrib
-	if limit < params.MinGasLimit {
-		limit = params.MinGasLimit
-	}
+	limit := max(parent.GasLimit()-decay+contrib, params.MinGasLimit)
 	// however, if we're now below the target (TargetGasLimit) we increase the
 	// limit as much as we can (parentGasLimit / 1024 -1)
 	if limit < params.TargetGasLimit {