Remove support for Xerox tools

[snomos]

The only maintainer is not among us anymore (and has not been for some years now), and the computers and systems around evolve. At some point it does not make sense to continue to support the Xerox tools anymore.

Removing support for them will clean up the make files quite a bit.

Any thoughts, @Trondtr @flammie others?

[flammie]

Sounds like a good idea, I haven't seen many active users of xerox version of tools lately that I remember and probably the tools aren't available without internet archive anymore either?

[snomos]

probably the tools aren't available without internet archive anymore either?

Good point, I believe you are right.

[trondtynnol]

I can mention that the only use I still have for xfst tools is interactive debugging of .twolc files in twolc. This is not as convenient using hfst-twolc. However, this does not really require the infrastructure to support xfst tools at all, and I do agree that xfst support makes the infrastructure files less readable.