Make onNegotiationNeeded conform to spec

[edaniels]

• Removes non-canon logic
• Addresses DataChannel negotiated peer connection's server.AddTrack doesn't always trigger client.OnTrack  #2774
• Effectively reverts ca05618

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.01%. Comparing base (d56bead) to head (be17ffd).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2792      +/-   ##
==========================================
+ Coverage   78.91%   79.01%   +0.10%     
==========================================
  Files          87       87              
  Lines        8208     8203       -5     
==========================================
+ Hits         6477     6482       +5     
+ Misses       1263     1255       -8     
+ Partials      468      466       -2     

Flag	Coverage Δ
go	80.64% <100.00%> (+0.12%)	⬆️
wasm	64.47% <100.00%> (+0.06%)	⬆️

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[edaniels]

@Sean-Der merging in since tests are passing and this fixes a probably nasty bug. We can revert if needed.