core: Revert "core: Allow parentless Media item"
core: Revert "core: Allow parentless Media item"

This reverts commit f7bf5909f5b1e1413ad0472a3a7f2fbf9e31edb3.

Pelagicore/Media-Manager-Rygel-patched,luisbg/rygel,Pelagicore/Media-Manager-Rygel-patched,Tarnyko/rygel,jku/rygel,GNOME/rygel,jku/rygel,GNOME/rygel,Tarnyko/rygel,Pelagicore/Media-Manager-Rygel-patched,GNOME/rygel