Remove the fifo plugin.  It's currently useless for the average user, and making it more presentable isn't something I'm willing to do before 0.12.  It will likely be added back after 0.12, along with some very experimental stuff to make it more usable.

git-svn-id: https://svn.musicpd.org/mpd/trunk@4472 09075e82-0dd4-0310-85a5-a0d7c8717e4f