[scripting] Make panels() not return the desktops() containments

It was filtering by isPanel() before port to QJSEngine:
033ad5ca

BUG: 435081

---

I've no idea if I should also filter by && !c->activity().isEmpty() like desktops() does.

---

Testing:

• Add a desktop widget
• qdbus org.kde.plasmashell /PlasmaShell showInteractiveConsole
• Run this script: https://gist.github.com/Zren/e1a1c1a851a3fa76216d056b8f95c144
• Apply patch and confirm panels() no longer prints org.kde.plasma.folder containments.