If buildtoc spots duplicated pods, that's an error condition, not a skip.

If buildtoc spots duplicated pods, that's an error condition, not a skip.

As of commit 9a8cc8a45d2bc98d, pod/buildtoc no longer scans ext/ for files
containing Pod. Hence it will no longer find the same file in both lib/ and
ext/, and hence the de-duplication code is no longer needed for correct
functionality. Duplicate Pods existing should never happen, so instead of
removing the code, it feels more useful to make it a build time error.

Add an editor block.