Remove unstable_expectedLoadTime option (#35051)

Follow up to #35022.

It's now replaced by the `defer` option.

Sounds like nobody is actually using this option, including Meta, so we
can just delete it.

DiffTrain build for [fa767dade6a22be994b5bf348fdabfc5301f72b4](https://github.com/facebook/react/commit/fa767dade6a22be994b5bf348fdabfc5301f72b4)