chore: Investigate `npm` issue for Samples Tests

[danielbankhead]

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

• https://github.com/googleapis/gaxios/pull/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:
  • https://github.com/googleapis/gaxios/blob/6fa80809656bafe85ce86602c190e4b2b7546576/.kokoro/samples-test.sh#L39
  • 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]

https://github.com/googleapis/synthtool/pull/2043