HTTP.jl: remove workarounds in WebSocketFix.jl?

[fonsp]

This PR added workarounds to Pluto's source code to fix an HTTP.jl bug: #580

A similar fix seems to have entered HTTP.jl some months later: JuliaWeb/HTTP.jl#638

So we might be able to remove https://github.com/fonsp/Pluto.jl/blob/main/src/webserver/WebSocketFix.jl

To test this, you need to send a lot of data over the WS at once. You could try:

@bind x @htl("""
<script>
currentScript.value = new Uint8Array(1000 * 1000 * 10) 
currentScript.dispatchEvent(new CustomEvent("input"))
</script>
""")

[fonsp]

We will be able to remove these after upgrading to HTTP 0.10: JuliaWeb/HTTP.jl#843