Dropping the "performance" section from the homepage & README.

[tomchristie]

Prompted by https://gitter.im/python-trio/general?at=60d99c99457e19611a227b52

So, I think it'd be valuable for us to drop the "performance" section from the homepage/README at this point in time.

Or at least considering moving it somewhere else, and/or being very careful about the phrasing we're using.

We do well on keeping Starlette slimline, but I'd also rather we weren't helping propagate the misguided line that microbenchmarking is a useful metric for evaluating frameworks.

If we do want to have something there then perhaps we can at least guide users towards why there are so many huge caveats with "benchmarking" against invented scenarios.

(From my POV the one genuine factor that I feel is noteworthy of praise in favour of Starlette is the low-overall-complexity of the stack. That's not something that's necessarily easy to provide metrics for, so it's a harder case to make. The fact that we perform decently against microbenchmarking is a result of that.)