In other words, what icon name do I have to put in dconf-editor/org/gnome/shell/extensions/icon-manager -> top-bar to get rid of the Removable Devices (and Ubuntu One) from sys tray?
You can possibly try this GNOME extension:
This extension adds a "status menu for accessing and unmounting removable devices."
This extension does not remove the removable devices from the messaging tray. You might want to try and hide the messaging tray to partially achieve the result you are looking for.
This is a twofold answer:
1. Removing the icon in the tray
This seems to be a development issue regarding Gnome-Shell usage on Ubuntu. A similar case is the redundant keyboard-layout icon, which appears both in the tray and panel. I reported the bug to Gnome, who explained:
So regarding removal of the tray icon, I am not sure there is an easy answer (it might be a bug or development goal for Ubuntu instead), but someone may surely be able to provide a hack.
2. Adding the icon in the panel
To add the removable device icon in the panel, you need the Removable Drive Menu extension.