Remove HTTP/2 pseudo headers from request primary headers

[JamesNK]

Kestrel now optimizes how it processes request static indexes - #24730

HTTP/2 pseudo headers will usually be sent as static indexes so there is no need to have them in the "primary headers" which are checked for first.

[Tratcher]

For context, you didn't do this in your PR because the KnownHeaders generated code kept breaking, right?

[JamesNK]

Yes. We'll see whether it still happens if it is the only change to KnownHeaders...