Drop libffi-devel from the cibuildwheel Linux before-all step

[bdraco]

What do these changes do?

Removes the libffi-devel install from
[tool.cibuildwheel.linux] before-all in pyproject.toml,
leaving the step responsible only for installing ccache.

History: libffi-devel was added in #75 (Dec 2024, "Fix wheel builds
when cffi needs to be built from source") because cffi was being
built from source during the wheel test environment install, and
cffi's build requires libffi headers. Today the wheel test chain
(requirements/test.txt: cython, covdefaults, pytest, pytest-cov,
pytest-xdist) and the build-system requires (expandvars, setuptools,
tomli) do not pull cffi. The only remaining cffi reference is
requirements/codspeed.txt, which is installed by the CodSpeed
benchmark job on a regular ubuntu-latest runner where libffi-dev is
already present and which does not use cibuildwheel.

Are there changes in behavior for the user?

No. CI-only change. The published wheels and the test command run by
cibuildwheel are unchanged; the only effect is a slightly faster
before-all (one fewer package installed, fewer yum/apt/apk fetches).

Related issue number

Follow-up cleanup spotted while reviewing #233 / #235.

Checklist

• I think the code is well written
• Unit tests for the changes exist -- N/A (CI workflow change; the
  effect is observable only by re-running the wheel-build matrix)
• Documentation reflects the changes -- N/A (no user-visible API)
• Add a new news fragment into the CHANGES/ folder

Agent run details (optional, for reviewers)

History trace:

• Introduced: git show 08091df (PR Fix wheel builds when cffi needs to be built from source #75, "Fix wheel builds when cffi
  needs to be built from source"; references the same fix in
  Implement support for the free-threaded build of CPython 3.13 multidict#1015).
• Reinforced: git show 36ec052 ("Revert building armv7l manylinux
  wheels / libcffi-dev is not available in the manylinux armv7
  images") -- confirming that the only consumer of libffi-devel
  was cffi.

Dep audit (current master):

• grep -l cffi requirements/*.txt -> only requirements/codspeed.txt.
• requirements/test.txt -> cython.txt + covdefaults + pytest +
  pytest-cov + pytest-xdist. None of these pull cffi transitively.
• pyproject.toml build-system requires -> expandvars, setuptools,
  tomli. None pull cffi.

Local checks:

• make doc-spelling: passes for the new fragment. Two pre-existing
  failures remain on master that are unrelated to this change
  (subdirectory in CHANGES/207.contrib.rst:1 and postfix in
  CHANGES.rst:168).
• Pre-commit hooks (toml check, codespell, etc.) pass.

The runtime effect cannot be exercised locally; verification is the
next Linux wheel-build job on this branch.

Drafted with Claude Code (Claude Opus 4.7); reviewed by @bdraco.

[codspeed-hq]

Merging this PR will not alter performance

✅ 4 untouched benchmarks

---

_{Comparing drop-libffi-devel-before-all (ff35ef4) with master (4179ce4)}

[bdraco]

Looks like ffi has long been fixed so we can merge this now