fix(lib): fully remove debug in production

[layershifter]

Fixes ##3484. This PR:

• fixes issue with not fully remove debug part in CJS builds, see lib/index.js, var _debug = require("./debug"); was present there
• removes debug() function as it was never used

[codecov-io]

Codecov Report

Merging #3485 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3485   +/-   ##
=======================================
  Coverage   99.82%   99.82%           
=======================================
  Files         172      172           
  Lines        2802     2802           
=======================================
  Hits         2797     2797           
  Misses          5        5

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[levithomason]

Released in [email protected].