test: refactor switch

`test-child-process-fork-net2.js` has a switch statement with 6 cases.
Each case uses `child.send()`, passing an object for the callback.
`child.send()` ignores the callback because it is not a function.
Removing the unused argument.

PR-URL: https://github.com/nodejs/node/pull/4870


Reviewed-By: Alexis Campailla <orangemocha@nodejs.org>