main: Stop owning the org.gnome.Panel name
This is a relict from the pre-3.0 days, when it was common to replace a running GNOME 2 session with gnome-shell. There has been no good reason to claim the name for many years, time to drop it.
If someone really wants to run gnome-panel alongside the shell, they probably know what they are doing. Or cannot be helped anyway.
Related: #3452