Removes unused dependency.

Used to minimize strophe-plugins which were inside the source tree and now npm handles them.

debian/control

@@ -3,7 +3,7 @@ Section: net
 Priority: extra
 Maintainer: Jitsi Team <dev@jitsi.org>
 Uploaders: Emil Ivov <emcho@jitsi.org>, Damian Minkov <damencho@jitsi.org>
-Build-Depends: debhelper (>= 8.0.0), yui-compressor
+Build-Depends: debhelper (>= 8.0.0)
 Standards-Version: 3.9.6
 Homepage: https://jitsi.org/meet