Remove -Wno-undefined-inline

[bricknerb]

See pybind/pybind11#5664 for context.

[bricknerb]

What should be the next step here?

[jiawen]

Sorry I'm a busy slammed with my day job at the moment.

I haven't fully grokked the linked discussion yet. But it looks like this change may break compatibility with users on older versions of Pybind11? If so we may need a guard. I'll take a look in a few days when I find cycles.

[henryiii]

You really should never be silencing this warning. It shows some of error in your code if an inline is not defined. It does not mean what @bricknerb thinks it means, though (see the linked issue). If you see this warning, the code should also not link. Which I think makes it even more important to remove. If something breaks, please report it as a bug.