feat: delete setup.py - migration complete

Removes setup.py as all configuration has been migrated to pyproject.toml
(PEP 621).

Verification confirms:
- Package installs correctly with pip install -e .
- All 142 entry points discoverable via importlib.metadata
- All tool configurations (pytest, pycodestyle, isort, importlinter) working
- Django checks pass for both LMS and CMS

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

setup.py

@@ -1,7 +0,0 @@
-"""  # lint-amnesty, pylint: disable=django-not-configured
-Setup script for the Open edX package.
-"""
-
-from setuptools import setup
-
-setup()