do not allow port-removal if the port would be re-added immediately
after that again because the main-delivery actually needs it.
As a side effect this prevents this crash:
* create a stereo-track, then remove one output
-> unhandled exception "AudioEngine::PortRegistrationFailure&"
The problem:
- the port is removed from the RCU ports list,
but Port::drop() (which calls jack_port_unregister) is only called
from the Port's destructor at some later time.
(because a reference to the port still exists elsewhere)
- the jack-port is not yet removed.
- meanwhile Delivery::configure_io comes along and notices that
there are more audio-buffers than ports and tries to re-register the port.
- but the port still exists in jack, so it fails and throws an exception
...which is not handled.
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