Decrement stream counts just after the stream is removed from stream_list
Decrement stream counts just after the stream is removed from stream_list

HTTP/2 active stream counts (`client_streams_in_count` and `client_streams_out_count`) are decremented in `Http2Stream::release_stream()`.
This is introduced by dc8be341578aa0fa900c84265d0a336414dfad9b. But these counts should be decremented just after the stream is
removed from `stream_list` in `Http2Stream::delete_stream()` like these counts are incremented just after the stream is added in
`stream_list`. Because when ATS sends END_STREAM flag or RST_STREAM frame, endpoints might open a new stream.

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