Fix a corner-case of pipe mode close-down processing, by simplifying
the logic and letting vca_close_session() and VBE_CloseFd() carry out
the last rites.

Closes: #746


git-svn-id: http://www.varnish-cache.org/svn/branches/2.1@5306 d4fa192b-c00b-0410-8231-f00ffab90ce4