The only reason that this check ever existed in the first place, is that originally there was a global `PDFJS.openExternalLinkInNewWindow` option which was then subsumed by the (more generic) `PDFJS.externalLinkTarget` option. (The `externalLinkTarget` has since been moved into a `PDFLinkService` option, as part of PDF.js version `2.0`.)
Hence, during the period where both `PDFJS.openExternalLinkInNewWindow` and `PDFJS.externalLinkTarget` existed side-by-side, there was a need to allow the former one to override the latter one (for backward compatibility purposes). However, that's no longer the case, and this extra `externalLinkTarget` check can now be removed.
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