Remove deprecated sage-cython script

[tobiasdiez]

It's deprectaed since v8.7 and is no longer in use in sage, so should be safe to be deleted.

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[orlitzky]

I'm stealing this script for another purpose in #39376 (which would be ready for review if I weren't waiting for a new repo under github.com/sagemath).

Basically, you can't compile external cython packages against sage-the-distro because cython will use sys.path from the system python, and thus all of sage is hidden from it. To work around that, I made sage-cython a thin wrapper that runs cython using the venv python.