Clean up or remove the FLINT QS spkg

[leif]

This is a follow-up to #11351; see this and this comment there.

At least FLINT 1.x ships with a (presumably more recent) version of the quadratic sieve program, so rather than cleanung up the separate flintqs package, one could change the FLINT spkg to also build and install this program.

[kcrisman]

See also #9544 and #12855, for reference.

[kcrisman]

See also #12173, upgrading FLINT to 2.3. Does the qsieve there do what we need?

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Also note #14202, which is orthogonal but also about qsieve, so it should be pointed out.

[Snark]

Puttins as need-info, since there's a pending question: does flint 2.3 make it possible to get flintqs out?

[jpflori]

I think FLINT 2.3 provides a quadratic sieve implementation but only for two limbs integers. See:

•  https://github.com/wbhart/flint2/blob/trunk/qsieve/doc/qsieve.txt

So I guess the answer is currently no. We'll have to wait until Bill or someone else ports the old code.

(FLINT 1.6 included a quadratic sieve to build as a separate executable, see:

•  http://www.flintlib.org/flint-1.6.pdf, section 18, page 64;

as did FLINT 1.5.2 (latest version included in Sage):

•  http://www.flintlib.org/flint-1.5.2.pdf, section 14, page 57.)