Remove QRE graphing from graphical interface

[tturocy]

The graphical interface offers the ability to plot the computed QRE correspondence.

This was useful at one time (and folded into Gambit functionality that was previously in a separate tool called PXI). However, these days tools like Jupyter notebooks (or other visualisation systems) provide much better functionality. Further, the inclusion of this plotting capability requires embedding a number of third-party wxWidgets extensions which are not actively maintained; they're not guaranteed to continue working with future wxWidgets versions (and indeed may well not work properly anymore anyway).

Therefore, it makes sense to remove the built-in plotting and instead look for ways to generate and export data for external tools, or possibly even to call those external tools directly if relevant.\