[docs] Delete obsolete ML guided optimisation doc (flutter#54237)

In 2021, we did some experiments with ML-guided optimisations in the
engine. Ultimately, these didn't provide much benefit, and much of the
information in the documentation is now out-of-date.

For those who need it, this can always be found in the git history.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style