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: https://github.com/nodejs/node/pull/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>