extractrc: Fix double-unescaping of `"`

I recently found that Krita hasn't been loading translations for UI strings containing the double quote " (see https://mail.kde.org/pipermail/kde-i18n-doc/2021-June/000583.html and https://invent.kde.org/-/snippets/1730 for context) and eventually I traced to this part of the script which I strongly suspect to be the root of the issue. What seems to have been happening is that " first got unescaped as ", which is then unescaped again to ". I do not how to test the whole POT extraction process though, so this has all been untested guesswork.

I suspect if this change is merged, we are going to see plenty of translations being updated to fix the incorrect unescaping, which would also mess up the translation workflow for some teams.