fs: remove unused argument from copyObject()

The fs function copyObject() had two arguments:
source and target. On the first line of the function it
assigned the target variable to:
arguments.length >= 2 ? target : {};

The function copyObject() was not called directly by
any test, but it is called in other fs functions. When it
was called it was only ever called with a single argument,
source. Thus I have removed the target argument and assigned
it to an empty object like it was being assigned to in the
original ternary operator.

PR-URL: nodejs#10041
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

lib/fs.js

@@ -56,8 +56,8 @@ function getOptions(options, defaultOptions) {
   return options;
 }
 
-function copyObject(source, target) {
-  target = arguments.length >= 2 ? target : {};
+function copyObject(source) {
+  const target = {};
   for (const key in source)
     target[key] = source[key];
   return target;