Delete "triangle" resuming fuzz tester

This deletes the ReactIncrementalTriangle test suite, which I originally
added back in 2017 when I was working on Fiber's "resuming" feature. It
was meant to simulate a similar scenario as Seb's "Sierpinski Triangle"
Fiber demo.

We eventually ended up removing resuming, but we kept this fuzz tester
around since it wasn't really harming anything. However, over the years,
we've had to make many small tweaks to decouple it from implementation
details, to the point that it doesn't test anything useful anymore. And
the thing that it originally tested has long since been removed.

If or when we do add back resuming, we would write a different fuzz
tester from scratch rather than build on this one.

So rather than continue to contrive ways to prevent it from breaking, I
propose we delete it.

We still have other fuzz testers for things like Suspense and context
propagation. Only this particular one has outlived its usefulness.