fs: removing unnecessary assignment

In `rethrow` function, instead of assigning to `err` and throwing
`err`, we can directly throw `backtrace` object itself.

lib/fs.js

@@ -46,8 +46,7 @@ function rethrow() {
       if (err) {
         backtrace.stack = err.name + ': ' + err.message +
                           backtrace.stack.substr(backtrace.name.length);
-        err = backtrace;
-        throw err;
+        throw backtrace;
       }
     };
   }