[RuntimeAsync] Remove JitOptimizeAwait knob (or make checked), eventually.

[VSadov]

Re: https://github.com/dotnet/runtimelab/pull/2951/files#r1932462336

Right now there is some value in having a release knob for the Await folding - primarily to see the impact on benchmarks.

The knob is also useful to validate that the optimization is indeed optional and everything works without it, although for that a nonrelease knob may be sufficient.

When we get closer to any kind of releasing, the knob will need to be removed or become nonrelease.