build: remove unused tools/cython_setup.py

The distutils/setuptools build driver is dead code since the move to
CMake. The Cython invocation now passes --cplus --3str directly on the
command line (CMakeLists.txt). Nothing imports or calls it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: linesight