Simplify bin/jsx to perform just the JSX transform.

We will continue using `bin/jsx-internal`, well, internally.

Note that this version no longer respects `@providesModule`, and it
doesn't do anything special with constants like `__DEV__`, so we can no
longer get to claim that `bin/jsx` can be used to build the core.

I'm happy about this, personally, because it demonstrates the flexibility
of Commoner.