remove support for the ECN test case

[marten-seemann]

The interop runner test case requires us to proactively set ECN bits, which we currently don't do (see #2789).

[codecov]

Codecov Report

Merging #2793 into master will decrease coverage by 0.81%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2793      +/-   ##
==========================================
- Coverage   86.92%   86.11%   -0.81%     
==========================================
  Files         133      133              
  Lines       10106     8956    -1150     
==========================================
- Hits         8784     7712    -1072     
+ Misses        990      913      -77     
+ Partials      332      331       -1     

Impacted Files	Coverage Δ
internal/testdata/cert.go	57.89% <0.00%> (-8.77%)	⬇️
internal/utils/buffered_write_closer.go	66.67% <0.00%> (-8.33%)	⬇️
internal/wire/handshake_done_frame.go	50.00% <0.00%> (-5.56%)	⬇️
http3/response_writer.go	70.00% <0.00%> (-5.00%)	⬇️
internal/handshake/header_protector.go	78.26% <0.00%> (-4.79%)	⬇️
internal/handshake/token_protector.go	54.29% <0.00%> (-4.69%)	⬇️
internal/protocol/connection_id.go	81.82% <0.00%> (-4.39%)	⬇️
closed_session.go	79.41% <0.00%> (-4.31%)	⬇️
http3/request_writer.go	61.87% <0.00%> (-3.26%)	⬇️
http3/frames.go	74.60% <0.00%> (-3.17%)	⬇️
... and 120 more

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[marten-seemann]

Codecov is going crazy again. No idea why it thinks that this would reduce code coverage...