Remove "darwin/386" pair as it has been obsoleted in Go 1.15

[eclipseo]

See golang/go#37610

[codecov]

Codecov Report

Merging #69 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #69   +/-   ##
=======================================
  Coverage   15.89%   15.89%           
=======================================
  Files          10       10           
  Lines         258      258           
=======================================
  Hits           41       41           
  Misses        212      212           
  Partials        5        5           

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