Move poppler and openjpeg back to core-deps
Epiphany no longer needs these for its flatpak-builder manifest. poppler in particular is risky since it doesn't have stable API/ABI. Let's reconsider again in the future if needed, but this wouldn't have been needed in the first place if we were building apps from gnome-build-meta, so maybe that will be ready next time another app wants to use one of these....