child_process: fix memory leak in .fork()

Entries in the `net.Server#_workers` array that is used to track handles
sent from the master to workers were not deleted when a worker exited,
resulting in a slow but inexorable memory leak.

PR-URL: https://github.com/nodejs/node/pull/15679
Fixes: https://github.com/nodejs/node/issues/15651


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>