comps: install xfce4-pulseaudio-plugin instead of xfce4-volumed and mixer

xfce4-volumed is no longer maintained, so switch to pulseaudio plugin.

Fixes QubesOS/qubes-issues#2321
Fixes QubesOS/qubes-issues#2291

conf/comps-qubes.xml

@@ -1142,8 +1142,7 @@
     <uservisible>false</uservisible>
     <packagelist>
       <packagereq>pavucontrol</packagereq>
-      <packagereq>xfce4-mixer</packagereq>
-      <packagereq>xfce4-volumed</packagereq>
+      <packagereq>xfce4-pulseaudio-plugin</packagereq>
       <packagereq type="default">alsa-utils</packagereq>
     </packagelist>
   </group>