Now that the tests have been codemodded, we can delete toFlushAndYield
et al from the codebase. See facebook#26285 for full context.

The mock implementation of Scheduler still exposes some (unstable-
prefixed) APIs that can be used for some edge cases, like when testing
the mock Scheduler itself. There are only a few tests that do this, and
even some of those could be rewritten to use `act` or `waitFor`.

(This only affects our own internal repo; it's not a public API.)

I think most of us agree this is a less confusing name. It's possible
someone will confuse it with `console.log`. If that becomes a problem
we can warn in dev or something.