Remove is_function_or_class helper footgun

[hauntsaninja]

This is a no-op change.

That function was not a good way to tell if something is a function or a class, since it basically only worked for async functions by accident (the parent of a suite / simple_stmt of async function body is a funcdef). Also not all async functions even have an async_funcdef

[github-actions[bot]]

diff-shades reports zero changes comparing this PR (df1257fd5c0feb2f5ff382f0230ebc40e0d2d76f) to main (db9c592967b976a16eccd500f3e2676cfff7f29d).

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[JelleZijlstra]

Thanks for all the improvements!