build: drop cpython-freethreading enable group

The cibuildwheel commit we pin (post-#2684) no longer recognizes
cpython-freethreading: 3.13t support was dropped and free-threading is the
default for 3.14+. Keeping the value triggers:

    cibuildwheel: Failed to parse enable group. Unknown enable group:
    cpython-freethreading.

Only cpython-prerelease is still needed (for cp315 / cp315t while 3.15 is
in beta).

Co-Authored-By: Claude <noreply@anthropic.com>

Author: art049

pyproject.toml

@@ -104,7 +104,7 @@ push = false
 
 [tool.cibuildwheel]
 build = "cp*{manylinux,macosx}*"
-enable = ["cpython-freethreading", "cpython-prerelease"]
+enable = ["cpython-prerelease"]
 test-extras = ["build", "test", "compat"]
 test-command = "pytest -v --ignore={project}/tests/benchmarks {project}/tests"