The flight-browser fixtures doesn't make sense. It also uses UMD buil…

…ds which are now 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 fixtures