dillon1000/react

Remove XHR support from Flight (#26827)

We currently support passing an XHR request to Flight for broader compat
and possibly better perf than `fetch()`. However, it's a little tricky
because ideally the RSC protocol is really meant to support binary data
too. XHR does support binary but it doesn't support it while also
streaming.

We could maybe support this only when you know it's going to be only
text streams but it has some limitations in how we can encode separators
if we can't use binary.

Nobody is really asking for this so we might as well delete it.