Remove support for is_async without a deprecation window

[vytas7]

Normally, we always give an adequate deprecation window when removing features, even if it is a major release.

However, the is_async parameter (used for hooks and media validation) represents an edge case where I very much doubt that anyone from the community has ever needed this flag.

What is more, very easy workarounds exist: wrapping the method in question inside an async def, or even easier, simply using inspect.markcoroutinefunction() (on CPython 3.12+) to mark the method as such. In addition, Cython 3.0 (on CPython 3.10+) no longer needs this flag, as it is able to convey the information to the interpreter via setting the appropriate code flags.