docs: remove "npm test-all" from CONTRIBUTING.md"

[sxa]

Spotted during #1108
npm test-all seems to have been removed from package.json in https://github.com/nodejs/node-core-utils/pull/670/changes#diff-7ae45ad102eab3b6d7e7896acd08c427a9b25b346470d7bc6507b6481575d519 but was left in the documentation. If something else should replace it then let me know, otherwise removing it and leaving npm test in there seems reasonable.

[sxa]

FYI @aduh95 as the author of the original PR. I don't seem to have permissions to assign you as an explicit reviewer.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.52%. Comparing base (2d1085f) to head (049dfda).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1109   +/-   ##
=======================================
  Coverage   64.52%   64.52%           
=======================================
  Files          45       45           
  Lines        7003     7003           
=======================================
  Hits         4519     4519           
  Misses       2484     2484           

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