Unregister Qt documentation when Qt version is removed

Keep in mind that

• multiple Qt versions can refer to the same documentation files (e.g. Qt 5.9.5 MSVC and Qt 5.9.5 MinGW from the same online installer)
• multiple Qt versions can refer to the same documentation namespaces, but different files (e.g. Qt 5.9.5 from an offline installer and Qt 5.9.5 from a custom build incl. documentation)
  • in that case the documentation files of the removed Qt version should no longer be referred to by QtHelp, so the documentation namespace(s) might need to be removed and re-added for a different documentation file