chore: Investigate `npm` issue for Samples Tests

[danielbankhead]

Investigate the need for npm i -g npm@10 in:

• feat!: Support Node 18+ #650 (.kokoro/pre-samples-test.sh)

Once determined, remove the .kokoro/pre-samples-test.sh file.

[danielbankhead]

Investigation notes:

• npm only appears to fail for samples in the npm link command, found here:

  • gaxios/.kokoro/samples-test.sh

    Line 39 in 6fa8080

    npm link ../

  • With an ENOENT: no such file or directory, lstat '/h/.npm-global' issue [example]
    • Attempting to create it leads to ENOTDIR: not a directory, lstat '/h/.npm-global/lib'
• npm i -g npm@10 works - even reinstalling the exact same version fixes the problem
  • See: https://github.com/googleapis/gaxios/pull/650/commits

[d-goog]

googleapis/synthtool#2043