ng_pipe: Remove node when all hooks are disconnected

This is the behavior described in the man page.

Signed-off-by: Martin Vahlensieck <git@academicsolutions.ch>

Discussed with: glebius
Reviewed by: markj
MFC after: 2 weeks
Pull Request: https://github.com/freebsd/freebsd-src/pull/1181

(cherry picked from commit bb2ab7a374b9d81f8368acc506c9700af20a22d6)