macOS package: drop support for Octave

The current Octave support is utterly broken (both in stable and unstable), it
crashes Octave. It relies on an unofficial Octave app for
macOS (https://octave-app.org), which is infrequently updated.

This commits drop support for Octave in the macOS package. We will now tell our
macOS+octave users to use the Homebrew Dynare package (which is maintained by
the Homebrew team, and is in reasonably good shape).