Cleanup unzipped function directories properly on OSX

[kenxu95]

Describe your changes and why you are making these changes

start_function_executor.sh errors on OSX operating systems because of the "${OP_PATH::-3}" expression.

We remove the need for this expression, and thus allow for cleanup of these expanded functions on Macs.

Related issue number (if any)

ENG-1375

Checklist before requesting a review

• I have created a descriptive PR title. The PR title should complete the sentence "This PR...".
• I have performed a self-review of my code.
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• If this is a new feature, I have added unit tests and integration tests.
• I have run the integration tests locally and they are passing.
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• All features on the UI continue to work correctly.
• Added one of the following CI labels:
  • run_integration_test: Runs integration tests
  • skip_integration_test: Skips integration tests (Should be used when changes are ONLY documentation/UI)