Unreference elements after priority queue Pop

[edaniels]

Fixes https://github.com/pion/webrtc/issues/2778

cc: @at-wat

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 72.43%. Comparing base (c7615af) to head (34a7c03).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #264      +/-   ##
==========================================
- Coverage   72.56%   72.43%   -0.13%     
==========================================
  Files          79       79              
  Lines        3648     3653       +5     
==========================================
- Hits         2647     2646       -1     
- Misses        867      870       +3     
- Partials      134      137       +3     

Flag	Coverage Δ
go	72.43% <100.00%> (-0.13%)	:arrow_down:
wasm	70.59% <100.00%> (-0.02%)	:arrow_down:

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