Delete reference to removed object

This prevents calling a C function with an old pointer from the JS
timeout callback which then crashes the GNOME Shell.
An upstream fix was also sent in which prevents the crash of GNOME
Shell but still this bug here needs to be fixed.

Fixes kgshank#110 kgshank#108

sound-output-device-chooser@kgshank.net/base.js

@@ -245,6 +245,7 @@ var SoundDeviceChooserBase = class SoundDeviceChooserBase{
 
     _deviceRemoved(control, id, dontcheck) {
         let obj = this._devices[id];
+        this._devices[id] = null;
         if(obj && obj.active) {
             _d("Removed: " + id);
             if(!dontcheck && this._canShowDevice(obj.uidevice, false)) {