http2: remove redundant write indirection

`nghttp2_stream_write_t` was not a necessary redirection layer
and came with the cost of one additional allocation per stream write.

Also, having both `nghttp2_stream_write` and `nghttp2_stream_write_t`
as identifiers did not help with readability.

PR-URL: https://github.com/nodejs/node/pull/17718


Reviewed-By: James M Snell <jasnell@gmail.com>