The new code relies on qz(..., 'real'), ordqz and ordeig, and returns a real
decomposition. The previous version was using Sims' qzdiv and returned a
complex decomposition.

As a consequence, we can drop options_.qzdiv, which was used to detect when
imaginary parts had to be dropped.

This code does not work on Octave for the time being, but this is acceptable
since it is only a fallback.