dropping support for kash? (RFC)

[Dima Pasechnik]

kash3 is an old closed-source program, to which Sage provides
interfaces for doing computations of Galois groups of number fields,
and integer factorization.

It is only available in form of the binary downloads, dated back to
2008 or even earlier.
Cf. https://www.kantmath.de/download.shtml

We very much doubt any current or future user of Sage has any use for it.
Integer factorization isn't something which is not covered in Sage
otherwise, and its Galois
group computations capabilities are now covered by GAP.
See https://github.com/sagemath/sage/issues/42523 and
https://github.com/sagemath/sage/pull/42524

If there are no objections by the end of the week, we'll proceed to remove it.

Dima