An idle qubit should still be considered idle even if it has a barrier

[garrison]

I noticed in the fragment of code for identifying idle qubits that it will identify a qubit as non-idle if it has a barrier, even if nothing else is on it:

https://github.com/Qiskit-Extensions/circuit-knitting-toolbox/blob/67f24cb4e8e067eebad1f351ce971df8dc115f61/circuit_knitting/utils/transforms.py#L170-L177

This is in contrast to the proposed similar method in Qiskit/qiskit#12470.