mixer-control: Don't emit output-update on default-sink set

mixer-control: Don't emit output-update on default-sink set

Don't emit the signal when setting. It will be emitted when the sink
actually changes. This makes things consistent with switching the
default source.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Origin: https://gitlab.gnome.org/GNOME/libgnome-volume-control/-/merge_requests/31
Bug: https://gitlab.gnome.org/GNOME/libgnome-volume-control/-/issues/34
Bug-Debian: https://bugs.debian.org/1126514
Applied-upstream: gnome-shell 50, commit:798230599950607ec285daecd070f5bcb28580e4

Gbp-Pq: Topic libgvc
Gbp-Pq: Name mixer-control-Don-t-emit-output-update-on-default-sink-se.patch