Use normal cfg values in examples

[madsmtm]

We started using cfg_aliases! in #2586, which I guess makes sense from a maintenance perspective (though I'll admit, I'm somewhat against it because it feels like an unnecessary dependency and adds build time).

However, its usage has crept into our examples, which I believe to be a bad idea, as the user will not be having those cfgs, meaning that they cannot directly copy our example code.

So I propose using the normal target_os = "macos"/feature = "x11" values in there.

[kchibisov]

I have a plan to remove all the CFGs all together in 0.31.