[chore] remove unnecessary Request pseudo-header removal

[Conduitry]

In #3572 to fix #3566 we started stripping HTTP/2 pseudo-headers from the incoming requests because node-fetch's Request implementation couldn't handle them. This may or may not have been possible to revert when we switched to undici and were only using node-fetch for its multipart formdata handle - but it should certainly be possible now that we're not using node-fetch at all.

My understanding is that there's currently something else amiss with Vite's --https mode, so I tested this with the Node adapter and a custom entrypoint defining an HTTP/2 server, as described in #3566, and things seemed to work fine.

This change does mean that event.request.headers will start having these HTTP/2 pseudo-headers present when an HTTP/2 server is being used, but this is probably safe - especially since we're no longer automatically copying incoming request headers into outgoing request headers for server-side fetches.

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