Remove flight-browser fixture (#28828)

The flight-browser fixtures doesn't make sense. It also uses UMD builds
which are being removed so we'd have to make it use esm.sh or something
and really it just won't work because it needs to be built by webpack to
not error. We could potentially shim the webpack globals but really the
right thing is to publish the esm version of react-server and use esm.sh
to load a browser only esm demo of react-server. This change removes the
flight-browser fixture