Remove createRootStrictEffectsByDefault flag (#28102)

There's no need to separate strict mode from strict effects mode any
more.

I didn't clean up the `StrictEffectMode` fiber flag, because it's used
to prevent strict effects in legacy mode. I could replace those checks
with `LegacyMode` checks, but when we remove legacy mode, we can remove
that flag and condense them into one StrictMode flag away.