coverage: remove stale Cython-only extensions #999

scripts/coverage.sh removes only dual-source Cython modules before reinstalling with CASS_DRIVER_NO_CYTHON=1. Stale binaries such as row_parser, obj_parser, numpy_parser, and cython_lz4 remain, keeping HAVE_CYTHON true and making local coverage incomplete and environment-dependent.

Remove all affected Cython extensions before the pure-Python reinstall.

Found while reviewing #950; unrelated to that PR.