Remove unused variable flagged by cython-lint 0.22

[ilgrad]

The Lint workflow installs cython-lint unpinned (uv pip install cython-lint --upgrade), and cython-lint 0.22.0 (released 2026-10-07) fails on main:

$ cython-lint src/
src/flint/types/fmpz.pxd:25:16: 'w' defined but unused (try prefixing with underscore?)

This makes the Lint job red on every PR opened since then (for example #433, which only touches .pyi files).

Cause. fmpz_set_pylong in src/flint/types/fmpz.pxd declares cdef ulong w but never uses it. It has been unused since the from_bytes/to_bytes rewrite in f349183; the w in ulong_from_little_endian just above is that function's own local. cython-lint 0.22.0 now checks unused locals in inline functions defined in .pxd files, which earlier releases did not report.

Fix. Delete the declaration.

Validation (Python 3.12, as in the Lint job):

	cython-lint 0.21.1	cython-lint 0.22.0
main (cb37c77)	pass	fail (the error above)
this branch	pass	pass

cython -3 -I src (Cython 3.3.0) still compiles fmpz.pyx, fmpq.pyx and fmpz_poly.pyx, which cimport fmpz.pxd. I did not run the full build or the test suite locally; the declaration was dead, so no generated code that runs changes.

[oscarbenjamin]

You can just do this in the other PR. We don't need separate PRs and CI runs for something like this.

[ilgrad]

Thanks, moved into #433.