Remove unused_tcp_port{,_factory} fixtures

[njsmith]

Closes gh-11

[codecov]

Codecov Report

Merging #15 into master will increase coverage by 4.51%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
+ Coverage   71.81%   76.32%   +4.51%     
==========================================
  Files           9        9              
  Lines         220      207      -13     
  Branches       18       17       -1     
==========================================
  Hits          158      158              
+ Misses         61       48      -13     
  Partials        1        1

Impacted Files	Coverage Δ
pytest_trio/plugin.py	63.56% <ø> (+5.81%)	⬆️

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