Remove async_hacks from the tokenizer

The async_hacks related functionality provides a specific compile mode that still allows treating ASYNC and AWAIT like soft keywords optionally. This is not used in the interpreter and it belongs to a version that has lost upstream support (and that would imply that someone is analyzing 3.6 code from 3.13 code) and it complicates the tokenizer logic, so we can remove it to clean things a bit.