build(pyproject): drop unused setuptools runtime dependency

[lukehsiao]

setuptools became a dependency in commit 249fcf8 ("Add setuptools to dependencies") because app.py imported pkg_resources and Rye-managed virtualenvs do not ship setuptools. That import was replaced in commit 852f501 ("Switch from pkg_resources to importlib.metadata in app.py, refs #2057"), so nothing in Datasette has consumed the dependency since. pluggy's load_setuptools_entrypoints() reads entry points through importlib.metadata despite its name, and the publish/package Dockerfile templates never reference setuptools.

Discovered using pyproject-udeps [1].

Tested: in a fresh uv venv, which ships without setuptools:

$ uv venv t && uv pip install --python t/bin/python .
$ t/bin/python -c "import setuptools"
ModuleNotFoundError: No module named 'setuptools'
$ t/bin/datasette --version
datasette, version 1.0a36
$ t/bin/datasette plugins
[]
$ t/bin/datasette --get /-/versions.json
{"ok": true, ...}

Ref: #2065
Ref: #2057