CB-12528 use fs instead of our own existsSync

[purplecabbage]

Platforms affected

Lib

What does this PR do?

Removes unneeded API

What testing has been done on this change?

Checklist

• Reported an issue in the JIRA database
• Commit message follows the format: "CB-3232: (android) Fix bug with resolving file paths", where CB-xxxx is the JIRA ID & "android" is the platform affected.
• Added automated test coverage as appropriate for this change.

[codecov-io]

Codecov Report

Merging #524 into master will decrease coverage by -0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #524      +/-   ##
==========================================
- Coverage    81.9%   81.88%   -0.02%     
==========================================
  Files          68       68              
  Lines        5504     5498       -6     
  Branches     1090     1090              
==========================================
- Hits         4508     4502       -6     
  Misses        996      996

Impacted Files	Coverage Δ
cordova-lib/src/cordova/util.js	97.2% <100%> (-0.08%)	❌
cordova-lib/src/util/npm-helper.js	95.65% <100%> (ø)	✅

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[stevengill]

It was deprecated for a time but then got undeprecated. Great find! nodejs/node#1592