Disable building of the SWIG binding for Octave

Disable building of the SWIG binding for Octave

The Debian Octave Group is preparing the transtion Octave5 ⇒ Octave 6 [1]
and mathgl is in the way.

Now, mathgl uses SWIG for building its binding for Octave.  However, SWIG
is currently incompatible with Octave 6 and neither the Octave upstream
authors [2] nor the SWIG upstream authors [3] are willing to fix the
problem.

The simple solution is to drop the Octave support of mathgl. At any rate,
dropping Octave for mathgl will have absolutely no effect in the package,
because the two *.oct files generated at building are not installed
anywhere. Even though mathgl build-depends on liboctave-dev, none of the
resulting binary package depend on Octave in any way.

[1] https://bugs.debian.org/976386
[2] https://savannah.gnu.org/bugs/?func=detailitem&item_id=59422
[3] https://github.com/swig/swig/issues/1893

Closes: #976381