DEP: drop dangling runtime dependency on setuptools

[neutrinoceros]

Found by running the following command from astropy/:

❯ uv tree --package setuptools --invert
Resolved 166 packages in 11ms
asdf-astropy v0.7.0
├── astropy v7.1.dev253+g4a55b10002 (extra: all)
│   └── asdf-astropy v0.7.0 (*)
├── astropy v7.1.dev253+g4a55b10002 (extra: dev-all) (*)
└── astropy v7.1.dev253+g4a55b10002 (extra: test-all) (*)
setuptools v75.6.0
└── astropy-sphinx-theme v1.1
    └── sphinx-astropy v1.9.1
        ├── astropy[confv2] v7.1.dev253+g4a55b10002 (extra: dev) (*)
        ├── astropy[confv2] v7.1.dev253+g4a55b10002 (extra: dev-all) (*)
        └── astropy[confv2] v7.1.dev253+g4a55b10002 (extra: docs) (*)

I find that this package doesn't actually require setuptools at runtime, so I assume it was listed by mistake when it should only have been a build-time requirement.

[neutrinoceros]

to be clear; only this part is problematic, setup_requires is fine in itself, but it's still redundant with pyproject.toml and should not be needed at all with python_requires = >=3.7