remove module-xenpv-sink

remove module-xenpv-sink

The module doesn't build any more[1], and when starting to investigate
the build failure, I asked the module author if he'd know something
about the breakage. He said that he didn't know about backward
compatibility problems with libxen, but more importantly, he said that
the module probably doesn't have any users[2]. It doesn't make sense to
keep maintaining a module that doesn't have users, so let's drop it.

[1] https://bugs.freedesktop.org/show_bug.cgi?id=98793
[2] https://lists.freedesktop.org/archives/pulseaudio-discuss/2016-November/027172.html