- Set the node proxy to NULL when it is destroyed externally
so that we don't try to destroy it a second time in finalize.
- Don't remove the dsp_listener, as this is already done inside
the node proxy destroy sequence (but keep removing the node
listener to avoid calling node_proxy_destroy() from inside
finalize())

After discussing things at the AGL May 2019 F2F meeting
and reflecting on the initial design of WirePlumber,
it became clear that it needed a fresh start.