npm rm <noargs> should globally remove the local pkg

(Creating this as a PR on isaac's request because there's no real reason not to land it. Needs a test and review but is otherwise good to go.)

Fix #4005 #6248

What's interesting is that the comment in the code seems to indicate
that this was *always* the intent.  But somewhere along the line, that
seems to have broken.

This makes 'npm link' be un-done by running 'npm unlink'.