Remove un-actionable warning about not finding an external symbol

[d-ronnqvist]

Bug/issue #, if applicable:

Summary

This removes an un-actionable warning. We've already removed a similar warning in #966. However, this warning is different in that there are real scenarios where we don't expect to resolve all external symbols, in which case this warning would be noise for expected behavior.

Dependencies

None.

Testing

None

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests Not applicable
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary Not applicable

[d-ronnqvist]

@swift-ci please test