remove the http3.DataStreamer

[marten-seemann]

This interface was never really useful: It allowed converting the HTTP stream into a bare QUIC stream in the HTTP handler (server-side, that is), but it didn't allow the equivalent thing on the client side.

It also doesn't look like there's any interest in this function. Searching GitHub for http3.DataStreamer yields 5 results across 4 repos, which have 6 GitHub stars (in total!).

[codecov]

Codecov Report

Merging #3435 (6e0d99a) into master (b5ef99a) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3435      +/-   ##
==========================================
- Coverage   85.69%   85.67%   -0.01%     
==========================================
  Files         135      135              
  Lines        9955     9947       -8     
==========================================
- Hits         8530     8522       -8     
  Misses       1048     1048              
  Partials      377      377              

Impacted Files	Coverage Δ
http3/response_writer.go	74.47% <ø> (-2.00%)	⬇️
http3/server.go	74.12% <100.00%> (-0.33%)	⬇️

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