Camera nodes do not get removed in 0.5

This is a regression on current master compared to 0.4.17: USB camera nodes don't get removed anymore from the graph after unplugging.

STR:

• have a USB camera connected
• check cameras with pw-dump | grep video, gst-device-monitor-1.0 Video/Source or a GUI like Helvum
• unplug and re-plug the camera
• check again and have two cameras, one not working when selected by a camera app like Snapshot or Cheese
• repeat and have three cameras