remove CircleCI build status from README

README.md

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 # QPACK
 
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 This is a minimal QPACK ([RFC 9204](https://datatracker.ietf.org/doc/html/rfc9204)) implementation in Go. It is minimal in the sense that it doesn't use the dynamic table at all, but just the static table and (Huffman encoded) string literals. Wherever possible, it reuses code from the [HPACK implementation in the Go standard library](https://github.com/golang/net/tree/master/http2/hpack).