gnu: gst-plugins-ugly: Remove libmad from the build environment.

Libmad is not used by gst-plugins-ugly since 1.12.0.

* gnu/packages/gstreamer.scm (gst-plugins-ugly)[inputs]: Remove libmad.

gnu/packages/gstreamer.scm

@@ -352,7 +352,6 @@ (define-public gst-plugins-ugly
     (inputs
      `(("gst-plugins-base" ,gst-plugins-base)
        ("liba52" ,liba52)
-       ("libmad" ,libmad)
        ("lame" ,lame)
        ("libcdio" ,libcdio)
        ("twolame" ,twolame)