cluster: remove NODE_UNIQUE_ID from env on startup

In case a worker would spawn a new subprocess with process.env, NODE_UNIQUE_ID
would have been a part of the env. Making the new subprocess believe it is a
worker, this would result in some confusion if the subprocess where to listen to
a port, since the server handle request would then be relayed to the worker.

This patch removes the NODE_UNIQUE_ID flag from process.env on startup so any
subprocess spawned by a worker is a normal process with no cluster stuff.