Most of sink input peak indicator streams are still connected alive at the time
their widgets are destroyed. Disconnect and unreference each one to stop memory
leak.
When sink input peak indicator stream is recreated, previous stream is only
disconnected but not unreferenced. Unreference it to stop memory leak.
Part-of: <https://gitlab.freedesktop.org/pulseaudio/pavucontrol/-/merge_requests/62>